diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 54b109b..cba5640 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -38,6 +38,9 @@ jobs:
       - name: Typecheck (electron)
         run: npm run typecheck:electron
 
+      - name: Localization and transcription behavior tests
+        run: npm run test:localization
+
       - name: Build Next.js
         run: npm run build
 
diff --git a/I18N_AND_TRANSCRIPTION_IMPLEMENTATION_PLAN.zh-CN.md b/I18N_AND_TRANSCRIPTION_IMPLEMENTATION_PLAN.zh-CN.md
new file mode 100644
index 0000000..5b847f4
--- /dev/null
+++ b/I18N_AND_TRANSCRIPTION_IMPLEMENTATION_PLAN.zh-CN.md
@@ -0,0 +1,674 @@
+# Rescript 汉化与原语种转录：手术刀式实施计划
+
+> 审计基线：2026-08-09；`jiezhengj/rescript` 与上游 `wassgha/rescript` 的 `main` 均指向 `d43e2d6afb8e4d2e82dc5d2b04e248f0435fc676`（Release v1.1.7）。
+>
+> 本文只规划实现，不包含功能代码改动。目标是尽量沿用现有结构，以小而独立、容易复核和回退的补丁完成简体中文界面、界面语言自动识别/切换，以及中文音频保持中文转录。
+>
+> 实施勘误（2026-08-09）：真实 Windows 验收发现项目固定的 Transformers.js 4.2 在省略 `language` 时会明确回退英语，其源码仍标注 `TODO: Implement language detection`。最终实现因此增加 Whisper 标准的首 token 语言探测，再以探测到的语言和 `task: "transcribe"` 正式解码；下文相关步骤已按验证后的方案修订。
+
+## 1. 结论先行
+
+当前的两个问题来源不同，不能共用一个“语言”设置：
+
+1. **界面一直是英文**：项目没有 i18n 层，React 文案、无障碍文案、运行进度、错误消息、Electron 原生菜单和更新对话框均直接写死为英文；`app/layout.tsx` 与 `app/global-error.tsx` 也固定使用 `lang="en"`。
+2. **中文音频得到英文文本**：项目实际上已经支持 `en/es/fr/de/zh` 五种转录语言，但默认值是 `en`。该值从 Zustand store 经 `useTranscriber` 发送给 Worker，最终作为 Whisper 的 `language: "en"` 解码约束。代码没有显式设置 `task: "translate"`，因此这不是一条明确的“翻译为英文”流水线，而是**默认英语解码约束造成了类似翻译的结果**。
+
+最小修复路线是：
+
+- 新增一层轻量、项目内置、只有 `en` 与 `zh-CN` 的界面本地化，不引入第三方 i18n 依赖；
+- 界面语言默认 `system`，根据浏览器/Electron 的系统语言识别，可手动切换并持久化；
+- 保留现有转录语言结构，新增 `auto` 偏好并改为新安装的默认值；
+- Whisper 始终显式传 `task: "transcribe"`；`auto` 时先把首 token 限制在模型的语言 token 集合中完成探测，再把结果（如 `zh`）传给正式解码；手动选中文时直接传 `language: "zh"`；
+- 自动语种模式不猜测 CTC 对齐模型，先使用现有 Whisper 词时间戳 + VAD/包络校正；手动选中文时继续使用项目已有的中文 XLS-R 对齐器；
+- 不增加“任意目标语言翻译”能力。Whisper 的“转录原文”和“翻译为英文”是不同任务，本需求只启用前者。
+
+## 2. 当前项目事实与根因
+
+### 2.1 技术与部署形态
+
+- Web/界面：Next.js 16 App Router、React 19、TypeScript、Tailwind。
+- 桌面端：同一套静态导出由 Electron 42 承载，目标为 macOS、Windows、Linux。
+- 状态：Zustand；项目和媒体保存在 IndexedDB；偏好保存在 localStorage。
+- 转录：Web Worker 内运行 `@huggingface/transformers@4.2.0`，Whisper Base/Small；另有 Parakeet v3。
+- 后处理：VAD、语言相关 CTC 强制对齐、说话人分离；均在本机运行。
+- Fork 状态：当前 fork 未包含自定义提交，与上游完全同步，适合从小型独立提交开始维护。
+
+### 2.2 已经存在、应当复用的转录语言能力
+
+当前代码已经具备以下基础，不应另起炉灶：
+
+- `lib/languages.ts`
+  - `TranscriptLanguage = "en" | "es" | "fr" | "de" | "zh"`；
+  - 语言标签、原生名称、旗帜、短代码；
+  - localStorage 键 `rescript.transcript-language`；
+  - 当前默认值 `DEFAULT_TRANSCRIPT_LANGUAGE = "en"`。
+- `components/ModelSelector.tsx`
+  - 模型菜单内已有转录语言子菜单；
+  - 非英语时会在 Whisper 模型按钮上显示语言短代码。
+- `lib/store.ts`、`lib/autosave.ts`、`lib/projects.ts`
+  - 转录语言已进入 store、项目保存和恢复流程。
+- `hooks/useTranscriber.ts`、`lib/types.ts`
+  - 语言随 `WorkerRequest` 发给转录 Worker。
+- `workers/transcription.worker.ts`
+  - Whisper 调用会传 `language: transcriptLanguage`；
+  - Parakeet 本身自动识别语言，但当前设置仍会影响后续对齐器选择。
+- `lib/alignModels.ts`
+  - 中文已经映射到 `onnx-community/wav2vec2-large-xlsr-53-chinese-zh-cn-ONNX`；
+  - 中文文字的 CTC 归一化模式已是 `cjk`。
+
+因此，本项目不缺“中文模型支持”，缺的是**正确的默认语义、明确的转录任务，以及更易发现的设置入口**。
+
+### 2.3 中文变英文的具体数据流
+
+```text
+lib/languages.ts
+  DEFAULT_TRANSCRIPT_LANGUAGE = "en"
+          ↓
+lib/store.ts
+  transcriptLanguage: "en"
+          ↓
+hooks/useTranscriber.ts
+  postMessage({ ..., language: "en" })
+          ↓
+workers/transcription.worker.ts
+  const transcriptLanguage = language ?? "en"
+  transcriber(audio, { ..., language: transcriptLanguage })
+          ↓
+Whisper 解码被约束为英语 token，中文语音表现为英文输出
+```
+
+当前 Worker 没有写 `task: "translate"`，但也没有写 `task: "transcribe"`。实施时应显式写出后者，避免模型配置或依赖升级改变默认行为。Hugging Face 的 Whisper 接口把 `task` 定义为 `transcribe` 或 `translate`，并把 `language` 定义为生成所用的语言 token；参见 [Whisper 官方文档](https://huggingface.co/docs/transformers/en/model_doc/whisper)。
+
+### 2.4 界面英文的分布
+
+用户可见英文不是只在组件文本节点里，还分布在以下边界：
+
+| 边界 | 当前位置 | 示例 |
+| --- | --- | --- |
+| 页面与主要组件 | `app/*.tsx`、`components/*.tsx` | Settings、Export、Tools、Speaker、Browse |
+| 无障碍/提示文字 | 多个组件的 `aria-label`、`title`、`placeholder` | Join clips、Zoom in、Search or create |
+| 进度消息 | `Editor`、`useTranscriber`、Worker | Detecting speech、Transcribing、Aligning words |
+| 可恢复错误 | `lib/ffmpeg.ts`、`lib/parseTranscript.ts`、`lib/store.ts` 等 | Could not extract audio、No words to export |
+| 相对日期 | `lib/projects.ts` | just now、3h ago |
+| 模型说明 | `lib/models.ts` | Faster download and transcription |
+| Electron 原生菜单 | `electron/menu.ts` | File、Open Project、Recent Projects |
+| 更新对话框 | `electron/updater.ts` | Update available、Restart、Later |
+| 根错误页 | `app/global-error.tsx` | Something went wrong、Try again |
+| HTML 语言/元数据 | `app/layout.tsx` | `lang="en"`、英文 title/description |
+
+控制台日志、Sentry stage、telemetry 事件名、内部异常诊断不属于界面文案，不应为了汉化而改名。
+
+## 3. 范围与非目标
+
+### 3.1 本次必须完成
+
+- 界面支持 English 与简体中文。
+- 默认跟随系统/浏览器语言；所有中文区域代码统一解析为 `zh-CN`，其他语言回退到 `en`。
+- 设置中可选：跟随系统、English、简体中文。
+- 手动选择持久化；选择“跟随系统”时响应浏览器 `languagechange`。
+- Web、macOS、Windows、Linux 的应用内界面使用同一有效语言。
+- Electron 菜单、自定义更新对话框与渲染器同步切换。
+- 转录语言支持“自动检测”，并将其设为未设置偏好时的默认值。
+- Whisper 始终执行原语种转录；中文音频在自动模式或手动中文模式下输出中文。
+- 用户仍可手动指定 `en/es/fr/de/zh`，用于短音频或自动识别不稳定时纠正模型。
+- 设置、项目保存和旧数据读取向后兼容。
+- 不新增运行时网络请求，不破坏离线模式。
+
+### 3.2 明确不做
+
+- 不把中文音频翻译为任意目标语言；不新增机器翻译模型。
+- 不翻译用户导入的 SRT/VTT/JSON 内容，只翻译应用 UI。
+- 不翻译品牌名、模型名、文件格式、快捷键和技术缩写，如 Rescript、Whisper、Parakeet、MP4、SRT、WebGPU。
+- 首期不做繁体中文词库；系统为 `zh-TW`/`zh-HK` 时暂时使用简体中文，并在设置中明确写“简体中文”。
+- 不重构编辑器、时间轴、导出、对齐或 IndexedDB 架构。
+- 不引入 `next-intl`、`react-i18next` 等依赖；当前只有两种 UI 语言且应用是 client-only，第三方框架收益不足以抵消改动面。
+- 不把 README、发行说明和官网营销内容纳入应用运行时汉化；可在功能稳定后另开文档 PR。
+
+## 4. 设计原则
+
+1. **两个语言域完全分离**
+   - `uiLocale`：应用按钮、菜单、提示所使用的语言。
+   - `transcriptLanguagePreference`：音频转录语言提示，和 UI 语言无关。
+   - 中文界面不能自动把所有音频强制设为中文；英文界面也不能强制英文转录。
+2. **有效语言在边界解析一次**
+   - UI 使用 `system | en | zh-CN` 偏好，解析为有效 `en | zh-CN`。
+   - 转录使用 `auto | en | es | fr | de | zh` 偏好；发送 Worker 前将 `auto` 解析为“无人工语言提示”，由 Worker 对音频执行模型原生首 token 探测。
+3. **传 key，不传英文句子**
+   - 进度和可预期错误尽量使用稳定 key/code；在渲染器按当前 UI 语言翻译。
+   - 控制台诊断仍保留英文，便于与上游和 Sentry 搜索结果一致。
+4. **不把翻译字典塞进编辑 store**
+   - UI locale 是安装偏好，不是媒体项目状态；用独立 Context/Hook 管理，避免污染现有编辑 undo/autosave 逻辑。
+5. **翻译 diff 与行为 diff 分开提交**
+   - 先修转录语义并测试；再加 i18n 基础设施；最后批量替换 UI 文案和桌面壳文案。
+
+## 5. 界面本地化方案
+
+### 5.1 最小数据模型
+
+新增纯 TypeScript 类型与解析函数：
+
+```ts
+export type UiLocale = "en" | "zh-CN";
+export type UiLocalePreference = "system" | UiLocale;
+
+export const DEFAULT_UI_LOCALE_PREFERENCE: UiLocalePreference = "system";
+
+export function resolveUiLocale(
+  preference: UiLocalePreference,
+  systemLanguages: readonly string[]
+): UiLocale;
+```
+
+解析规则：
+
+- 手动 `en`/`zh-CN` 直接使用；
+- `system` 时按 `navigator.languages` 的优先顺序寻找已支持语言：先遇到 `zh-*` 就解析为 `zh-CN`，先遇到 `en-*` 就解析为 `en`，不支持的语言继续向后查找；
+- 没有任何已支持语言时回退 `en`；
+- Electron 初次创建菜单时用 `app.getLocale()` 做同样解析；渲染器加载后再通过 IPC 发送最终有效语言，确保手动偏好覆盖系统值。
+
+偏好键建议为 `rescript.ui-locale`，只存 `system | en | zh-CN`，不要存解析后的系统语言。这样系统语言变化后，“跟随系统”仍然有效。
+
+### 5.2 轻量字典与 API
+
+新增 `lib/i18n.ts`，使用扁平 key，避免引入运行时依赖：
+
+```ts
+const en = {
+  "settings.title": "Settings",
+  "settings.language": "Interface language",
+  "transcription.language.auto": "Auto-detect",
+  "progress.transcribing": "Transcribing…",
+  "export.downloadFile": "Download {name}",
+} as const;
+
+const zhCN: Record<keyof typeof en, string> = {
+  "settings.title": "设置",
+  "settings.language": "界面语言",
+  "transcription.language.auto": "自动检测",
+  "progress.transcribing": "正在转录…",
+  "export.downloadFile": "下载 {name}",
+};
+```
+
+要求：
+
+- 以英文 key 集合约束中文词条完整性，缺 key 在 TypeScript 编译期失败；
+- `t(key, params?)` 仅做简单命名插值，不引入 ICU 解析器；
+- 数字、日期、相对时间使用 `Intl.NumberFormat`、`Intl.DateTimeFormat`、`Intl.RelativeTimeFormat`，避免拼接 `3h ago`；
+- 未知 key 在开发环境抛错或 console.error，生产环境回退英文；
+- 文案函数可在 React 之外调用，但不直接读取 Zustand。
+
+建议新增 `components/I18nProvider.tsx`：
+
+- lazy initializer 从 localStorage + `navigator.languages` 得到首屏语言，避免先渲染英文再闪成中文；
+- 提供 `{ locale, preference, setPreference, t }`；
+- 监听 `storage`，同步同源多个标签页；
+- preference 为 `system` 时监听 `languagechange`；
+- 更新 `document.documentElement.lang` 与 `document.title`；
+- 调用 `window.rescriptDesktop?.setUiLocale(locale)` 同步 Electron 主进程。
+
+`app/page.tsx` 在动态 `Editor` 外包一层 Provider。Provider 本身仍是 client component，不改变当前 client-only 编辑器架构。
+
+### 5.3 设置界面
+
+在 `components/SettingsMenu.tsx` 新增两个清晰分区：
+
+1. **界面语言 / Interface language**
+   - 跟随系统 / System
+   - English
+   - 简体中文
+2. **转录语言 / Transcript language**
+   - 自动检测 / Auto-detect（推荐）
+   - English、Español、Français、Deutsch、中文
+   - 辅助说明：“自动检测会保留音频的原语言；识别不准时可手动指定。”
+
+模型菜单中现有 `LanguageSection` 保留，因为用户通常在导入媒体前决定转录语言。将选项列表抽成一个小型可复用组件/数据源，设置菜单和模型菜单共享，不复制状态或语言数组。
+
+模型按钮在自动模式显示 `AUTO`，不要像当前英语默认值一样完全隐藏。这样用户能立即看出应用不会强制英语。
+
+Parakeet v3 的能力要诚实呈现：
+
+- Parakeet ASR 后端自身自动识别语种，现有 `parakeet.js` 调用不接受语言参数；
+- 手动语言只能作为后续对齐器提示，不能承诺控制 Parakeet 输出；
+- 在 Parakeet 被选中时显示“自动识别；手动语言仅用于词对齐”的短说明，或禁用强制语言入口；
+- 中文“确保原文输出”的验收以 Whisper Base/Small 为准。
+
+### 5.4 HTML、字体与根错误页
+
+- `app/layout.tsx`
+  - 保留 Next 静态 metadata 的英文基线；运行时由 Provider 更新 title；
+  - `<html lang>` 的初始值仍可为 `en`，但在交互前脚本中只设置 lang，不渲染文案；
+  - 不需要下载中文 WebFont。Geist 只包含 Latin，中文交给现有 system-ui 回退，可减少包体和字体闪动。
+- `app/global-error.tsx`
+  - 该组件替换 RootLayout，不能依赖 Provider；直接调用纯 `load/resolve/t` 函数生成中英文错误页；
+  - 同步设置正确的 `<html lang>`。
+
+### 5.5 文案覆盖规则
+
+首期必须覆盖所有用户能看到或屏幕阅读器能读到的文字：
+
+- 可见按钮、标题、说明、空状态；
+- `aria-label`、`title`、`placeholder`、图片 `alt`；
+- `alert`、`confirm`、对话框；
+- 进度、网络错误、导入/导出错误；
+- 模型说明、最近项目相对日期；
+- Electron 菜单、更新对话框。
+
+不翻译：
+
+- `console.warn/error` 诊断文字；
+- Sentry stage 名、telemetry event/property；
+- 程序员断言，例如“Popover components must be used…”；
+- 文件扩展名、模型 ID、品牌名。
+
+建议术语表：
+
+| English | 简体中文 | 说明 |
+| --- | --- | --- |
+| Transcript | 转录文本 | 指最终文字内容 |
+| Transcription | 转录 | 指处理过程 |
+| Transcript language | 转录语言 | 不写“翻译语言”，避免能力误导 |
+| Speaker | 说话人 | 与说话人分离术语一致 |
+| Cut | 剪除 | 时间轴移除内容的动作 |
+| Delete | 删除 | 删除所选词/片段 |
+| Restore | 恢复 | 还原已删除内容 |
+| Split | 分割 | 创建片段边界 |
+| Join clips | 合并片段 | 移除分割点 |
+| Filler words | 语气词 | 包括“嗯、呃、…” |
+| Export | 导出 | 生成媒体或文稿 |
+| Start over | 重新开始 | 回到导入页 |
+
+### 5.6 Electron 与各操作系统
+
+新增 `electron/locale.ts`，仅负责桌面主进程的有效 locale 和桌面文案。它不依赖 React，也不读取 renderer localStorage。
+
+IPC 最小扩展：
+
+- `types/rescript-desktop.d.ts`：增加 `setUiLocale(locale: "en" | "zh-CN")`；
+- `electron/preload.ts`：发送 `ui:set-locale`；
+- `electron/main.ts`：严格校验值，更新主进程 locale，并重建应用菜单；
+- `electron/menu.ts`：菜单模板从 locale 取标签，保留现有 role、accelerator、command，不改行为；
+- `electron/updater.ts`：更新完成时从主进程 locale 取 Restart/Later/title/message/detail。
+
+原生菜单需要覆盖手写的 File 项，也应给 Electron role 项显式提供中英文 label，保证“手动切换中文”在英文操作系统上同样生效。role 本身不变，快捷键与系统行为不变。
+
+Windows 安装器可作为同一批次的低风险配置补充：在 `package.json` 的 `nsis` 下只加入 English 与 Simplified Chinese 的 installer language，并启用多语言安装器；macOS 系统对话框、文件选择器和 Linux 桌面环境对话框由操作系统本地化，不应由应用伪造。需要以当前 electron-builder 版本的官方配置名为准后再提交，避免凭旧版配置猜测。
+
+## 6. 转录原语种方案
+
+### 6.1 类型拆分，但保留现有命名和文件
+
+不要把 `auto` 当成真实语种传给模型或对齐器。建议在 `lib/languages.ts` 增加：
+
+```ts
+export type TranscriptLanguage = "en" | "es" | "fr" | "de" | "zh";
+export type TranscriptLanguagePreference = "auto" | TranscriptLanguage;
+
+export const DEFAULT_TRANSCRIPT_LANGUAGE_PREFERENCE = "auto";
+```
+
+为减少连锁改名：
+
+- Zustand 字段可继续叫 `transcriptLanguage`，但类型改为 `TranscriptLanguagePreference`；
+- WorkerRequest 的 `language` 改为可选的具体 `TranscriptLanguage`，绝不接收字符串 `auto`；
+- `useTranscriber` 是偏好值到 Worker 参数的唯一转换边界。
+
+### 6.2 持久化兼容
+
+沿用 localStorage 键 `rescript.transcript-language`：
+
+- 没有旧值：返回新的默认 `auto`；
+- 旧值为 `en/es/fr/de/zh`：视为用户明确选择，原样保留；
+- 新值 `auto`：正常读取；
+- 非法值：回退 `auto`。
+
+不升级 IndexedDB `DB_VERSION`：字段不是索引，记录是结构化克隆对象，允许直接保存新字符串。旧项目的具体语言值仍合法；缺字段时回退 `auto`。旧项目打开时本来就跳过重新转录，因此不会改变其已有文字。
+
+这个迁移策略能修复绝大多数“从未手动选语言”的用户，同时不会把明确选择英语的用户悄悄改成自动。
+
+### 6.3 Worker 调用的唯一行为改动
+
+在 `hooks/useTranscriber.ts`：
+
+```ts
+const preference = store.transcriptLanguage;
+const language = preference === "auto" ? undefined : preference;
+```
+
+构造消息时，`auto` 必须**省略** `language` 属性，而不是传 `"auto"` 或 `undefined` 字符串。
+
+在 `workers/transcription.worker.ts`：
+
+```ts
+const detectedLanguage = transcriptLanguage ?? await detectWhisperLanguage(
+  transcriber,
+  longestSpeechProbe
+);
+
+const asrOptions = {
+  chunk_length_s: 29,
+  stride_length_s: 5,
+  return_timestamps: "word",
+  no_repeat_ngram_size: 4,
+  repetition_penalty: 1.05,
+  task: "transcribe",
+  language: detectedLanguage,
+};
+```
+
+必须删除 Worker 中的 `language ?? "en"`。关键语义：
+
+- 自动：先以 `<|startoftranscript|>` 为前缀，只允许模型生成语言 token，取得 `zh/en/...`；再以 `task=transcribe, language=<detected>` 输出原语言；
+- 中文：`task=transcribe, language=zh`，明确输出中文转录；
+- 英语：`task=transcribe, language=en`；
+- 代码中永远不使用 `task=translate`。
+
+语言选项提取为 `lib/transcriptionOptions.ts` 的纯函数，使测试能直接断言“自动探测出的任意 Whisper 语言代码都继续使用 transcribe、任何模式都不是 translate”。真正的模型探测另由安装客户端端到端测试覆盖。
+
+### 6.4 自动模式与强制对齐
+
+当前 `transcriptLanguage` 同时用于 Whisper 解码和 CTC 对齐模型选择。自动探测虽然能给出可靠语种，但该结果只存在于 Worker 单次任务中；为避免扩大项目持久化和下载行为，自动模式仍不默认套用语言专用 CTC 对齐器。
+
+首期采用最小、安全降级：
+
+- `runWhisper`、`runParakeet`、`refineWordTimestamps`、`forceAlign` 的 language 参数允许缺省；
+- 自动模式不预热/下载任何语言专用 CTC 对齐器；
+- 自动模式继续运行已有的：
+  1. Whisper/Parakeet 原始词时间戳；
+  2. `alignWordsToSpeech` 的 VAD/响度包络校正；
+  3. 语气词占位插入；
+  4. 说话人分离；
+- 手动 `zh` 仍预热并运行现有中文 XLS-R CTC 对齐器；其他手动语言行为保持不变。
+
+这种取舍比“从输出字符猜语种后下载对齐器”更可靠。仅凭拉丁文字很难区分英语、西班牙语、法语和德语；猜错对齐模型可能比不做 CTC 更坏。若后续确实需要“自动识别后仍强制对齐”，可把本次已经验证的 Whisper 首 token 探测结果显式纳入项目记录与对齐器生命周期，但那属于独立的下载/持久化行为扩展。
+
+### 6.5 进度和错误的本地化边界
+
+Worker 不能使用 React Context。不要把当前 UI locale 固化进一个长时间运行的 Worker；否则用户在转录中切换语言时，后续消息仍是旧语言。
+
+建议把 Worker 的进度改为稳定 stage：
+
+```ts
+type ProgressStage =
+  | "loadingSpeechModel"
+  | "detectingSpeech"
+  | "transcribing"
+  | "loadingAlignerFromCache"
+  | "downloadingAligner"
+  | "aligningWords"
+  | "identifyingSpeakers"
+  | "gpuFallback";
+
+type WorkerResponse =
+  | { type: "progress"; stage: ProgressStage; value: number | null }
+  | ...;
+```
+
+`useTranscriber` 将 stage 存入 store，显示组件用当前 `t()` 翻译。对于网络/模型等可预期错误，增加少量 error code；未知异常仍携带英文 fallback 并上报 Sentry。这样既能即时切换语言，又不翻译开发诊断。
+
+如果必须进一步压缩首个 PR，可先只把进度 stage 化，把罕见底层错误保留英文；但验收前必须覆盖普通用户能触发的导入、网络、媒体、导出错误。
+
+## 7. 精确文件改动清单
+
+### 7.1 新增文件（建议 4 个）
+
+| 文件 | 作用 |
+| --- | --- |
+| `lib/i18n.ts` | UI locale 类型、解析、存取、字典、插值和 Intl helpers |
+| `components/I18nProvider.tsx` | React locale 状态、事件监听、html/title/IPC 同步、`useI18n` |
+| `lib/transcriptionOptions.ts` | 从转录语言偏好生成可测试的 Whisper `{ task, language? }` |
+| `electron/locale.ts` | Electron 主进程 locale、菜单/更新对话框文案 |
+
+如单个 `lib/i18n.ts` 超过约 400 行，再拆为 `lib/i18n/{core,messages}.ts`；不要一开始为两个 locale 建复杂目录层级。
+
+### 7.2 转录行为改动（核心、小范围）
+
+| 文件 | 精确改动 |
+| --- | --- |
+| `lib/languages.ts` | 增加 preference/auto；默认改 auto；持久化校验兼容旧值；增加 Auto 显示信息 |
+| `lib/types.ts` | WorkerRequest 的 language 变为可选具体语种；Progress 使用 stage |
+| `lib/store.ts` | store 字段类型接受 auto；reset/hydrate 默认 auto；进度状态接受 stage |
+| `hooks/useTranscriber.ts` | auto → 省略 language；使用纯 options helper；进度按 stage 保存 |
+| `workers/transcription.worker.ts` | 删除英语 fallback；auto 用 Whisper 首 token 探测实际语种；正式解码显式 `task: "transcribe"` 和探测/手动 language；auto 跳过语言专用 aligner |
+| `lib/projects.ts` | 项目字段接受 preference；缺失/非法值回退 auto；不升级 DB |
+| `lib/autosave.ts` | 无结构改动，仅类型随 store 变化；确认仍保存 preference |
+| `components/ModelSelector.tsx` | 增加 Auto；trigger 显示 AUTO；复用语言选择 UI；说明 Parakeet 限制 |
+| `components/SettingsMenu.tsx` | 增加转录语言设置入口和简短解释 |
+
+`lib/alignModels.ts` 已经能对 `undefined` 返回 null，原则上只需类型/注释微调，不应重写映射。
+
+### 7.3 React UI 汉化
+
+必须逐文件只替换用户可见字符串，不做顺手重构：
+
+| 文件 | 主要文案范围 |
+| --- | --- |
+| `app/layout.tsx` | lang boot、运行时标题基线 |
+| `app/global-error.tsx` | 崩溃标题、说明、重试按钮、lang |
+| `app/page.tsx` | 挂载 I18nProvider；loading 的 aria 文案（如补充） |
+| `components/UploadScreen.tsx` | 拖放/浏览、准备状态、隔离错误、最近项目、三步说明、alerts |
+| `components/SettingsMenu.tsx` | 设置、外观、隐私、链接、新增的两种语言设置 |
+| `components/ModelSelector.tsx` | 模型/来源/语言菜单、aria、模型说明 |
+| `components/Editor.tsx` | 导入提醒、媒体错误、进度、Undo/Redo/Export |
+| `components/TopBar.tsx` | 重新开始及提示 |
+| `components/TranscriptPanel.tsx` | 导入、显示删除词、纠正、分割提示、跟随播放头 |
+| `components/TranscriptToolsMenu.tsx` | 工具、语气词/静音批处理、title/aria |
+| `components/SpeakerLabel.tsx` | 说话人命名、搜索、新建、移动、菜单 |
+| `components/Timeline.tsx` | 播放、前后跳、分割/删除/恢复、缩放、修剪、合并提示 |
+| `components/ExportDialog.tsx` | tabs、格式/分辨率、说明、渲染进度、下载/重试、错误 |
+| `components/ImportTranscriptOption.tsx` | 导入状态、错误和文件选择说明 |
+| `components/DesktopAppBanner.tsx` | 桌面端推荐、平台下载、关闭 |
+| `components/SocialLinks.tsx` | 无障碍标签；品牌名不翻译 |
+| `components/LogoLoader.tsx` | Loading aria |
+
+`components/Popover.tsx` 内用于程序员的 context 错误不翻译。
+
+### 7.4 非组件用户文案
+
+| 文件 | 处理方式 |
+| --- | --- |
+| `lib/models.ts` | label 保留模型名；description 改为翻译 key 或由 UI 映射 |
+| `lib/projects.ts` | `formatRelativeTime` 接收 locale，改用 Intl.RelativeTimeFormat |
+| `lib/ffmpeg.ts` | 用户可恢复错误改为 error code；内部诊断不改 |
+| `lib/parseTranscript.ts` | 空文件、格式错误等改为 error code |
+| `lib/serializeTranscript.ts` | 无内容可导出等改为 error code |
+| `hooks/useDesktopMenu.ts` | confirm/alert 本地化；console 保留英文 |
+| `electron/menu.ts` | 本地化所有菜单 label，不改 role/accelerator/command |
+| `electron/updater.ts` | 本地化更新对话框，不改 updater 流程 |
+| `electron/preload.ts` | 增加 locale IPC 方法 |
+| `electron/main.ts` | 校验 locale、同步菜单/更新器 |
+| `types/rescript-desktop.d.ts` | 补充 bridge 类型 |
+| `package.json` | 可选：Windows NSIS 英/简中语言；不新增 npm 依赖 |
+
+## 8. 实施顺序与提交拆分
+
+### 阶段 A：先固定转录行为
+
+1. 为转录偏好增加 `auto`，未设置用户默认 auto。
+2. 新增纯 `transcriptionOptions` helper。
+3. Worker 在 auto 时先执行 Whisper 语言 token 探测，再以探测语言和显式 `task: "transcribe"` 正式解码。
+4. auto 跳过 CTC；手动语言保持现有对齐路径。
+5. 更新项目持久化校验与语言单元测试。
+
+建议提交：`fix(transcription): preserve detected source language`
+
+这一提交只解决“中文变英文”，即使后续汉化需要返工，也能独立合并和回退。
+
+### 阶段 B：加入轻量 i18n 核心和设置
+
+1. 新增 `lib/i18n.ts` 与 key 完整性测试。
+2. 新增 Provider/Hook，默认 system，支持 persistence、storage、languagechange。
+3. 在 Settings 增加界面语言与转录语言入口。
+4. 更新 html lang 和根错误页。
+
+建议提交：`feat(i18n): add system-aware English and Simplified Chinese locales`
+
+### 阶段 C：逐表面替换 React 用户文案
+
+按页面分 2～3 个提交，避免一个超大 diff：
+
+1. 上传页、设置、模型选择；
+2. 编辑器、转录、说话人、时间轴；
+3. 导出、错误、无障碍文案。
+
+每个提交只做 `string → t(key)`，不调样式、不改业务逻辑。中文长度引起的布局修正单独提交，便于审查。
+
+### 阶段 D：桌面壳与跨平台收口
+
+1. 新增主进程 locale 模块和 IPC。
+2. 本地化菜单与 updater 对话框。
+3. 如官方配置验证通过，再加入 Windows 双语安装器。
+4. 在 macOS/Windows/Linux 打包产物中人工验证。
+
+建议提交：`feat(electron): localize native menus and update dialog`
+
+### 阶段 E：测试、文档和发布
+
+1. 执行完整 lint/typecheck/build。
+2. 运行纯函数测试与真实中文音频验收。
+3. 在 README 增加简短的 Language 行为说明；详细中文 README 可另做。
+4. 先发布 prerelease，验证自动更新和不同系统 locale，再发布正式版。
+
+## 9. 测试计划
+
+### 9.1 自动化测试
+
+新增/扩展：
+
+- `tests/i18n-test.ts`
+  - `zh`、`zh-CN`、`zh-Hans`、`zh-HK` → `zh-CN`；
+  - `en-US`、`fr-FR`、空数组 → `en`；
+  - 手动 preference 覆盖系统语言；
+  - 英中 key 完整一致；
+  - 插值缺参、未知 key 和英文 fallback；
+  - 相对时间在两种 locale 下不含硬编码英文片段。
+- `tests/languages-test.ts`
+  - `auto` 是合法 preference，但不是具体 `TranscriptLanguage`；
+  - 缺偏好默认 auto；
+  - 旧五种语言仍合法；
+  - 顺序为 Auto + 现有五种语言。
+- `tests/transcription-options-test.ts`
+  - auto：`{ task: "transcribe" }`，无 `language` key；
+  - zh：`{ task: "transcribe", language: "zh" }`；
+  - en/es/fr/de 同理；
+  - 没有任何分支返回 `translate`；
+  - auto 的 `alignModelFor(undefined)` 为 null，zh 仍选中文 aligner。
+- `tests/projects-test.ts`
+  - 旧项目具体语言读取不变；
+  - 缺字段/非法字段回退 auto；
+  - 保存/读取 auto 不需要 DB 升级。
+
+当前 CI 没有执行这些脚本式测试。最小做法是在 `package.json` 增加一个只运行上述快速纯函数测试的脚本，并在 `.github/workflows/ci.yml` 的 typecheck 后执行；不要顺手启用所有历史重型测试。
+
+### 9.2 构建验证
+
+必须通过现有检查：
+
+```bash
+npm ci
+npm run lint
+npx next typegen
+npx tsc --noEmit
+npm run typecheck:electron
+npm run build
+npm run build:electron
+npm run export:desktop
+```
+
+额外确认：
+
+- 不新增依赖，因此 `package-lock.json` 理论上不应发生依赖树变化；
+- 静态导出仍能从 `app://` 加载 Worker 与字典；
+- CSP/COOP/COEP 和 SharedArrayBuffer 行为不变；
+- 首次模型下载与离线缓存行为不变。
+
+### 9.3 真实音频验收矩阵
+
+至少准备 10～30 秒、发音清晰的中文和英文各一段；中文最好同时包含标点可推断的完整句子。
+
+| 模型 | 设置 | 音频 | 预期 |
+| --- | --- | --- | --- |
+| Whisper Base | Auto | 中文 | 输出中文，不出现整段英文翻译 |
+| Whisper Small | Auto | 中文 | 输出中文，不出现整段英文翻译 |
+| Whisper Base | 中文 | 中文 | 输出中文，并执行中文 CTC 对齐 |
+| Whisper Base | English | 中文 | 允许出现英语约束结果；这是用户显式选择 |
+| Whisper Base | Auto | 英文 | 输出英文 |
+| Whisper Base | 中文 | 很短中文 | 输出中文，验证手动提示纠正短片段 |
+| Parakeet | Auto | 其支持语种 | 保持后端自动识别；不承诺中文覆盖 |
+
+同时检查：
+
+- partial transcript 与最终 transcript 语种一致；
+- 中文词/字块均有非负、递增时间戳；
+- 时间轴删除、分割、导出字幕不因中文无空格而失败；
+- SRT/VTT/TXT/MD 导出保持 UTF-8；
+- 重新打开保存项目后中文不乱码；
+- 首次离线缓存后断网重试仍能转录。
+
+### 9.4 跨平台 UI 验收矩阵
+
+| 环境 | 系统语言 | 偏好 | 预期 |
+| --- | --- | --- | --- |
+| Chrome/Edge | 中文 | 跟随系统 | 首屏简中，无英文闪屏 |
+| Chrome/Edge | 英文 | 跟随系统 | 英文 |
+| Chrome/Edge | 英文 | 简体中文 | 简中；刷新后保持 |
+| Chrome/Edge | 中文 | English | 英文；刷新后保持 |
+| macOS Electron | 中文/英文各一次 | 跟随/手动各一次 | 页面、菜单、更新框一致 |
+| Windows Electron | 中文/英文各一次 | 跟随/手动各一次 | 页面、菜单、更新框一致 |
+| Linux Electron | 中文/英文各一次 | 跟随/手动各一次 | 页面与菜单一致 |
+
+重点人工检查 560×400 上传窗口和窄屏：中文文案不能撑破 Settings、ModelSelector、ExportDialog、时间轴工具栏。优先允许换行或略增 popover 宽度，不缩小字号。
+
+## 10. 风险与控制
+
+| 风险 | 影响 | 控制措施 |
+| --- | --- | --- |
+| 把 UI 语言当音频语言 | 中文界面转录英文音频时错误 | 两套类型、存储键和设置完全分离 |
+| auto 仍落回 en | 中文继续变英文 | 测试对象中断言没有 language key；删除所有 `?? "en"` |
+| 忘记 task | 依赖配置变化导致行为漂移 | helper 永远返回 `task: "transcribe"` 并测试 |
+| auto 使用错误 CTC | 时间戳被错误语言模型破坏 | auto 首期跳过语言专用 CTC |
+| Parakeet 设置语义误导 | 用户以为可强制输出中文 | UI 标出其 auto-only ASR 限制 |
+| Worker 内保存旧 locale | 转录中切换语言后进度混杂 | Worker 发 stage/code，渲染器即时翻译 |
+| 中文文案更宽/更高 | 小窗和菜单溢出 | 逐屏视觉测试；布局修复单独提交 |
+| Electron 启动菜单短暂错语 | 手动偏好需等 renderer 同步 | 初始用 app.getLocale，renderer mount 后立即 IPC 重建 |
+| 旧偏好被错误迁移 | 明确选英语的用户被改 auto | 保留所有已有合法具体值；仅“无值”默认 auto |
+| 上游频繁变化导致冲突 | Fork 难维护 | 不改组件结构，不全局格式化，按边界拆小提交 |
+
+## 11. “手术刀式”约束清单
+
+实施 PR 必须遵守：
+
+- 不添加 i18n npm 包；
+- 不更换 Zustand、Worker、模型或对齐器；
+- 不修改 Word/TimeRange/Speaker 等编辑核心数据结构；
+- 不升级 IndexedDB DB_VERSION；
+- 不更改 localStorage 旧键含义，只扩展合法值；
+- 不更改模型下载 ID、dtype、VAD、chunk/stride、重复惩罚等已调优参数；
+- 除新增 `task` 和可选 `language` 外，不改 Whisper 生成参数；
+- 不在汉化提交中重排 JSX、改 className 或运行全库格式化；
+- 不翻译日志/telemetry/Sentry 标识；
+- 每个阶段都可单独 revert；
+- 每次同步上游后先跑转录 options 单元测试，防止上游重新引入英语 fallback。
+
+## 12. 完成定义
+
+满足以下条件才算完成：
+
+1. 新用户在中文系统上首次打开即看到简体中文；英文系统看到英文。
+2. 设置可在“跟随系统 / English / 简体中文”间即时切换，刷新和重启后语义正确。
+3. Web 与 Electron 主界面、菜单、更新对话框、进度、普通错误、无障碍文案语言一致。
+4. 未设置转录偏好时显示 Auto，而不是隐式 English。
+5. 中文音频使用 Whisper Base/Small + Auto 时输出中文；手动中文同样输出中文。
+6. Worker 的 Whisper options 明确为 `task: "transcribe"`，自动模式不含 `language`，代码中无 `task: "translate"`。
+7. 自动模式不会误用英语 CTC；手动中文仍使用现有中文 CTC 对齐器。
+8. 旧的具体语言偏好和旧项目可无迁移打开。
+9. 所有现有 CI 构建检查和新增纯函数测试通过。
+10. 不增加运行时依赖、网络请求或模型下载；除用户手动指定语言时已有的对齐模型外，离线与缓存行为不变。
+
+## 13. 建议的后续增强（不阻塞本次）
+
+- 若需要自动模式也使用语言专用 CTC：研究并测试 Transformers.js 4.2.0 返回 Whisper 语言 token 的可靠路径，再把“检测到的具体语种”作为 Worker complete 元数据返回。
+- 增加繁体中文 `zh-TW` 词库，而不是对简体词条做机械转换。
+- 把官网、README、发行说明和下载页做多语言版本。
+- 为可视组件引入 Playwright screenshot/aria smoke test；当前仓库没有 UI 自动化框架，不应塞进首个修复 PR。
+- 如果未来支持三种以上 UI 语言或复杂复数规则，再评估 `next-intl`/ICU；届时可保留现有 key，不需要重翻文案。
diff --git a/app/global-error.tsx b/app/global-error.tsx
index 3e5b1d4..91acffd 100644
--- a/app/global-error.tsx
+++ b/app/global-error.tsx
@@ -2,6 +2,12 @@
 
 import { useEffect } from "react";
 import { reportError } from "@/lib/sentry";
+import {
+  loadUiLocalePreference,
+  resolveUiLocale,
+  systemLanguages,
+  translate,
+} from "@/lib/i18n";
 
 /**
  * Last-resort boundary for a crash that escaped the editor's own error handling.
@@ -18,12 +24,17 @@ export default function GlobalError({
   error: Error & { digest?: string };
   unstable_retry: () => void;
 }) {
+  const locale = resolveUiLocale(
+    loadUiLocalePreference(),
+    systemLanguages()
+  );
+  const t = (key: Parameters<typeof translate>[1]) => translate(locale, key);
   useEffect(() => {
     reportError(error, "render");
   }, [error]);
 
   return (
-    <html lang="en">
+    <html lang={locale}>
       <body
         style={{
           margin: 0,
@@ -35,7 +46,7 @@ export default function GlobalError({
           color: "var(--fg)",
         }}
       >
-        <title>Rescript — something went wrong</title>
+        <title>{t("globalError.title")}</title>
         <style>{`
           :root { --bg: #fafafa; --fg: #18181b; --muted: #71717a; --btn: #18181b; --btnfg: #fff; }
           @media (prefers-color-scheme: dark) {
@@ -44,11 +55,10 @@ export default function GlobalError({
         `}</style>
         <main style={{ padding: "2rem", textAlign: "center", maxWidth: "34rem" }}>
           <h1 style={{ fontSize: "17px", fontWeight: 600, margin: 0 }}>
-            Something went wrong
+            {t("globalError.heading")}
           </h1>
           <p style={{ color: "var(--muted)", fontSize: "13px", marginTop: "0.75rem" }}>
-            The editor hit an unexpected error. Your saved projects are still on
-            this device — reloading should bring them back.
+            {t("globalError.body")}
           </p>
           <button
             type="button"
@@ -65,7 +75,7 @@ export default function GlobalError({
               color: "var(--btnfg)",
             }}
           >
-            Try again
+            {t("common.retry")}
           </button>
         </main>
       </body>
diff --git a/app/layout.tsx b/app/layout.tsx
index 4bb0655..b3b52ab 100644
--- a/app/layout.tsx
+++ b/app/layout.tsx
@@ -64,6 +64,7 @@ export const metadata: Metadata = {
 
 /** Apply stored appearance before paint to avoid a light→dark flash. */
 const appearanceBootScript = `(function(){try{if(localStorage.getItem("rescript.appearance")==="dark")document.documentElement.classList.add("dark")}catch(e){}})();`;
+const localeBootScript = `(function(){try{var p=localStorage.getItem("rescript.ui-locale")||"system";var l=p;if(p==="system"){var a=navigator.languages&&navigator.languages.length?navigator.languages:[navigator.language];l="en";for(var i=0;i<a.length;i++){var v=(a[i]||"").toLowerCase();if(v==="zh"||v.indexOf("zh-")===0){l="zh-CN";break}if(v==="en"||v.indexOf("en-")===0){l="en";break}}}document.documentElement.lang=l==="zh-CN"?"zh-CN":"en"}catch(e){}})();`;
 
 export default function RootLayout({
   children,
@@ -82,6 +83,11 @@ export default function RootLayout({
           strategy="beforeInteractive"
           dangerouslySetInnerHTML={{ __html: appearanceBootScript }}
         />
+        <Script
+          id="locale-boot"
+          strategy="beforeInteractive"
+          dangerouslySetInnerHTML={{ __html: localeBootScript }}
+        />
       </head>
       <body className="min-h-full">
         {children}
diff --git a/app/page.tsx b/app/page.tsx
index 2d6d17c..cd00a1d 100644
--- a/app/page.tsx
+++ b/app/page.tsx
@@ -1,6 +1,7 @@
 "use client";
 
 import dynamic from "next/dynamic";
+import I18nProvider from "@/components/I18nProvider";
 
 // The editor relies on browser-only APIs (Workers, WebAssembly, WebGPU),
 // so it is rendered exclusively on the client.
@@ -14,5 +15,9 @@ const Editor = dynamic(() => import("@/components/Editor"), {
 });
 
 export default function Home() {
-  return <Editor />;
+  return (
+    <I18nProvider>
+      <Editor />
+    </I18nProvider>
+  );
 }
diff --git a/components/DesktopAppBanner.tsx b/components/DesktopAppBanner.tsx
index e41e6d3..da68ec3 100644
--- a/components/DesktopAppBanner.tsx
+++ b/components/DesktopAppBanner.tsx
@@ -9,6 +9,7 @@ import {
   type Platform,
 } from "@/lib/platform";
 import { usePlatform } from "@/hooks/usePlatform";
+import { useI18n } from "./I18nProvider";
 
 const DISMISSED_STORAGE_KEY = "rescript:desktop-banner-dismissed";
 
@@ -87,6 +88,7 @@ function PlatformIcon({ platform }: { platform: Platform }) {
  * the user dismisses it.
  */
 export default function DesktopAppBanner() {
+  const { t } = useI18n();
   const platform = usePlatform();
   const [dismissed, setDismissed] = useState(
     () => isElectron || readDismissed(),
@@ -100,9 +102,9 @@ export default function DesktopAppBanner() {
     <div className="relative flex h-11 shrink-0 items-center justify-center gap-3 border-b border-blue-200/60 bg-gradient-to-b from-blue-50/70 to-blue-100/50 pr-11 pl-3 text-[13px] text-blue-900 dark:border-blue-400/15 dark:from-blue-950/40 dark:to-blue-900/25 dark:text-blue-100">
       <p className="truncate">
         <span className="hidden sm:inline">
-          Want faster transcription and exports?{" "}
+          {t("banner.faster")}{" "}
         </span>
-        Get the Rescript desktop app
+        {t("banner.getDesktop")}
       </p>
       <a
         href={downloadUrlFor(platform)}
@@ -111,15 +113,15 @@ export default function DesktopAppBanner() {
         className="flex h-7 shrink-0 items-center gap-1.5 rounded-full bg-white/70 pr-3.5 pl-3 text-[12px] font-medium text-blue-700 ring-1 ring-blue-300/60 transition hover:bg-white hover:ring-blue-400/70 dark:bg-white/10 dark:text-blue-100 dark:ring-white/15 dark:hover:bg-white/15 dark:hover:ring-white/25"
       >
         <PlatformIcon platform={platform} />
-        {label ? `Download for ${label}` : "Download"}
+        {label ? t("banner.downloadFor", { platform: label }) : t("common.download")}
       </a>
       <button
         onClick={() => {
           setDismissed(true);
           writeDismissed();
         }}
-        title="Dismiss"
-        aria-label="Dismiss"
+        title={t("banner.dismiss")}
+        aria-label={t("banner.dismiss")}
         className="absolute right-2.5 flex h-6 w-6 cursor-pointer items-center justify-center rounded-md text-blue-500 transition hover:bg-blue-500/10 hover:text-blue-800 dark:text-blue-300 dark:hover:bg-white/10 dark:hover:text-blue-100"
       >
         <X size={14} />
diff --git a/components/Editor.tsx b/components/Editor.tsx
index 8c4ceb9..b5e0431 100644
--- a/components/Editor.tsx
+++ b/components/Editor.tsx
@@ -1,6 +1,6 @@
 "use client";
 
-import { useCallback, useEffect, useRef, useState } from "react";
+import { useCallback, useEffect, useRef } from "react";
 import { Group, Panel, Separator, useDefaultLayout } from "react-resizable-panels";
 import { useEditorStore } from "@/lib/store";
 import { getCutRanges, isWordCutOut } from "@/lib/edits";
@@ -23,7 +23,6 @@ import MediaPreview from "./MediaPreview";
 import Timeline from "./Timeline";
 import ExportDialog from "./ExportDialog";
 import { Download, Redo2, Undo2 } from "lucide-react";
-import LogoLoader from "./LogoLoader";
 import SettingsMenu from "./SettingsMenu";
 import ModelSelector, {
   LanguageSection,
@@ -33,11 +32,7 @@ import ModelSelector, {
 import ImportTranscriptOption from "./ImportTranscriptOption";
 import { MODEL_ORDER } from "@/lib/models";
 import { isTypingTarget } from "@/lib/keyboard";
-
-/** How long the desktop mode-change overlay stays up. Matches the macOS
- *  `setBounds(..., animate)` duration plus a small buffer so the layout
- *  underneath isn't revealed mid-resize. */
-const WINDOW_MODE_OVERLAY_MS = 380;
+import { useI18n } from "./I18nProvider";
 
 /** Transcript and preview split, resizable in both orientations. Wide screens
  *  put the transcript first (left of the preview); stacked screens lead with
@@ -128,6 +123,7 @@ function EditorWorkspace() {
 }
 
 export default function Editor() {
+  const { t } = useI18n();
   const status = useEditorStore((s) => s.status);
   const videoFile = useEditorStore((s) => s.videoFile);
   const skipTranscription = useEditorStore((s) => s.skipTranscription);
@@ -140,9 +136,6 @@ export default function Editor() {
   const redo = useEditorStore((s) => s.redo);
   const setExportOpen = useEditorStore((s) => s.setExportOpen);
 
-  const [modeTransitioning, setModeTransitioning] = useState(false);
-  const wasIdle = useRef(status === "idle");
-
   // File › Open Project… reaches the same picker the upload screen uses, from
   // anywhere in the app.
   const menuInputRef = useRef<HTMLInputElement>(null);
@@ -156,7 +149,7 @@ export default function Editor() {
       const { source, pendingTranscript } = useEditorStore.getState();
       if (source === "import") {
         if (!pendingTranscript) {
-          alert("Choose a transcript file from the source menu first.");
+          alert(t("editor.chooseTranscript"));
           return;
         }
         loadVideo(file, {
@@ -167,7 +160,7 @@ export default function Editor() {
       }
       loadVideo(file);
     },
-    [loadVideo]
+    [loadVideo, t]
   );
 
   // The pipeline needs SharedArrayBuffer, so a file picked before the page is
@@ -188,7 +181,7 @@ export default function Editor() {
     (file: File | undefined) => {
       if (!file) return;
       if (!detectMediaKind(file)) {
-        alert("Please choose a video or audio file.");
+        alert(t("editor.chooseMedia"));
         return;
       }
       if (!isolated) {
@@ -197,7 +190,7 @@ export default function Editor() {
       }
       startMenuFile(file);
     },
-    [isolated, startMenuFile]
+    [isolated, startMenuFile, t]
   );
 
   // Daily-active signal: reports the launch, then again on each day rollover so
@@ -247,18 +240,11 @@ export default function Editor() {
     })();
   }, [videoFile, skipTranscription, transcribe]);
 
-  // The desktop shell opens as a small upload window and grows once the
-  // three-pane editor takes over (and shrinks back on "start over").
-  // Cover the swap with a brief overlay so the layout reflow isn't visible
-  // while the window animates between sizes.
+  // The main process uses this mode only to preserve the native close behavior
+  // (close project first, then close the upload window). It no longer resizes
+  // the window; user-chosen bounds are persisted by Electron instead.
   useEffect(() => {
-    const idle = status === "idle";
-    window.rescriptDesktop?.setWindowMode(idle ? "compact" : "expanded");
-    if (!isElectron || wasIdle.current === idle) return;
-    wasIdle.current = idle;
-    setModeTransitioning(true);
-    const timer = window.setTimeout(() => setModeTransitioning(false), WINDOW_MODE_OVERLAY_MS);
-    return () => window.clearTimeout(timer);
+    window.rescriptDesktop?.setWindowMode(status === "idle" ? "compact" : "expanded");
   }, [status]);
 
   // Global shortcuts: space = play/pause, ⌘Z / ⇧⌘Z = undo / redo, S = split,
@@ -349,7 +335,7 @@ export default function Editor() {
       {status === "idle" ? (
         <>
           {isElectron && <TopBar>
-            <ModelSelector groupLabel="Transcript source">
+            <ModelSelector groupLabel={t("model.transcriptSource")}>
               {MODEL_ORDER.map((id) => (
                 <ModelOption key={id} id={id} />
               ))}
@@ -369,7 +355,7 @@ export default function Editor() {
             <button
               onClick={undo}
               disabled={!canUndo}
-              title="Undo (⌘Z)"
+              title={t("editor.undo")}
               className="flex h-8 w-8 items-center justify-center rounded-lg text-zinc-500 transition hover:bg-zinc-100 hover:text-zinc-800 cursor-pointer disabled:cursor-not-allowed disabled:opacity-30 disabled:hover:bg-transparent dark:text-zinc-400 dark:hover:bg-zinc-800 dark:hover:text-zinc-200"
             >
               <Undo2 size={16} />
@@ -377,7 +363,7 @@ export default function Editor() {
             <button
               onClick={redo}
               disabled={!canRedo}
-              title="Redo (⇧⌘Z)"
+              title={t("editor.redo")}
               className="flex h-8 w-8 items-center justify-center rounded-lg text-zinc-500 transition hover:bg-zinc-100 hover:text-zinc-800 cursor-pointer disabled:cursor-not-allowed disabled:opacity-30 disabled:hover:bg-transparent dark:text-zinc-400 dark:hover:bg-zinc-800 dark:hover:text-zinc-200"
             >
               <Redo2 size={16} />
@@ -389,7 +375,7 @@ export default function Editor() {
               className="flex ml-1 h-8 items-center gap-1.5 rounded-full bg-zinc-900 px-4 text-[13px] font-medium text-white transition hover:bg-zinc-700 cursor-pointer disabled:cursor-not-allowed disabled:opacity-40 dark:bg-zinc-100 dark:text-zinc-900 dark:hover:bg-zinc-200"
             >
               <Download size={14} />
-              Export
+              {t("editor.export")}
             </button>
             <div className="mx-1 h-5 w-px bg-zinc-200 dark:bg-zinc-700" />
             <SettingsMenu />
@@ -398,15 +384,6 @@ export default function Editor() {
           <Timeline />
         </>
       )}
-      {modeTransitioning && (
-        <div
-          className="absolute inset-0 z-50 flex items-center justify-center bg-zinc-50 dark:bg-zinc-950"
-          aria-busy="true"
-          aria-live="polite"
-        >
-          <LogoLoader size={44} />
-        </div>
-      )}
       {isElectron && (
         // Present in the layout tree (not display:none) so the native menu's
         // click() reliably opens the picker.
diff --git a/components/ExportDialog.tsx b/components/ExportDialog.tsx
index 2289c2b..0f66f1d 100644
--- a/components/ExportDialog.tsx
+++ b/components/ExportDialog.tsx
@@ -25,6 +25,8 @@ import {
   type TranscriptDocFormat,
 } from "@/lib/serializeTranscript";
 import { useCutRanges } from "@/hooks/useCutRanges";
+import { useI18n } from "./I18nProvider";
+import { localizeRuntimeMessage } from "@/lib/i18n";
 
 type ExportTab = "video" | "audio" | "transcript" | "subtitles";
 
@@ -58,6 +60,7 @@ const SUBTITLE_FORMATS: { value: SubtitleFormat; label: string }[] = [
 ];
 
 export default function ExportDialog() {
+  const { t } = useI18n();
   const open = useEditorStore((s) => s.exportOpen);
   const setOpen = useEditorStore((s) => s.setExportOpen);
   const videoFile = useEditorStore((s) => s.videoFile);
@@ -255,33 +258,33 @@ export default function ExportDialog() {
   }[] = [
     {
       id: "video",
-      label: "Video",
+      label: t("export.video"),
       icon: Film,
       disabled: isAudioProject,
       title: isAudioProject
-        ? "Video export isn’t available for audio-only projects"
+        ? t("export.videoUnavailable")
         : undefined,
     },
     {
       id: "audio",
-      label: "Audio",
+      label: t("export.audio"),
       icon: Music,
       disabled: !hasAudioTrack,
-      title: !hasAudioTrack ? "This file has no audio track" : undefined,
+      title: !hasAudioTrack ? t("export.noAudio") : undefined,
     },
     {
       id: "transcript",
-      label: "Transcript",
+      label: t("export.transcript"),
       icon: FileText,
       disabled: !hasWords,
-      title: !hasWords ? "Transcribe or import a transcript first" : undefined,
+      title: !hasWords ? t("export.noWordsFirst") : undefined,
     },
     {
       id: "subtitles",
-      label: "Subtitles",
+      label: t("export.subtitles"),
       icon: Captions,
       disabled: !hasWords,
-      title: !hasWords ? "Transcribe or import a transcript first" : undefined,
+      title: !hasWords ? t("export.noWordsFirst") : undefined,
     },
   ];
 
@@ -298,7 +301,7 @@ export default function ExportDialog() {
       >
         <div className="mb-4 flex items-center justify-between">
           <h2 className="text-lg font-semibold text-zinc-900 dark:text-zinc-50">
-            Export
+            {t("export.title")}
           </h2>
           <button
             onClick={() => setOpen(false)}
@@ -312,7 +315,7 @@ export default function ExportDialog() {
         <div
           className="mb-5 grid grid-cols-4 gap-0.5 rounded-xl bg-zinc-100 p-0.5 dark:bg-zinc-800"
           role="tablist"
-          aria-label="Export type"
+          aria-label={t("export.type")}
         >
           {tabs.map(({ id, label, icon: Icon, disabled, title }) => {
             const selected = activeTab === id;
@@ -340,25 +343,29 @@ export default function ExportDialog() {
 
         {(activeTab === "video" || activeTab === "audio") && (
           <div className="mb-5 grid grid-cols-3 gap-2 text-center">
-            <Stat label="Original" value={formatTime(duration)} />
-            <Stat label="Cuts" value={String(cuts.length)} />
-            <Stat label="Edited" value={formatTime(editedDuration)} accent />
+            <Stat label={t("export.statOriginal")} value={formatTime(duration)} />
+            <Stat label={t("export.statCuts")} value={String(cuts.length)} />
+            <Stat label={t("export.statEdited")} value={formatTime(editedDuration)} accent />
           </div>
         )}
 
         {activeTab === "video" && (
           <div className="mb-5 space-y-4">
             <OptionGroup
-              label="Format"
+              label={t("export.format")}
               value={videoFormat}
               options={VIDEO_FORMATS}
               disabled={exporting}
               onChange={setVideoFormatOption}
             />
             <OptionGroup
-              label="Resolution"
+              label={t("export.resolution")}
               value={resolution}
-              options={VIDEO_RESOLUTIONS}
+              options={VIDEO_RESOLUTIONS.map((option) =>
+                option.value === "original"
+                  ? { ...option, label: t("export.original") }
+                  : option
+              )}
               disabled={exporting}
               onChange={setResolutionOption}
             />
@@ -368,7 +375,7 @@ export default function ExportDialog() {
         {activeTab === "audio" && (
           <div className="mb-5">
             <OptionGroup
-              label="Format"
+              label={t("export.format")}
               value={audioFormat}
               options={AUDIO_FORMATS}
               disabled={exporting}
@@ -380,13 +387,17 @@ export default function ExportDialog() {
         {activeTab === "transcript" && (
           <div className="mb-5 space-y-3">
             <OptionGroup
-              label="Format"
+              label={t("export.format")}
               value={transcriptFormat}
-              options={TRANSCRIPT_FORMATS}
+              options={TRANSCRIPT_FORMATS.map((option) =>
+                option.value === "txt"
+                  ? { ...option, label: t("export.plainText") }
+                  : option
+              )}
               onChange={setTranscriptFormat}
             />
             <p className="text-xs text-zinc-500 dark:text-zinc-400">
-              Speaker-labeled text with cuts removed. No timestamps.
+              {t("export.transcriptHelp")}
             </p>
           </div>
         )}
@@ -394,21 +405,20 @@ export default function ExportDialog() {
         {activeTab === "subtitles" && (
           <div className="mb-5 space-y-3">
             <OptionGroup
-              label="Format"
+              label={t("export.format")}
               value={subtitleFormat}
               options={SUBTITLE_FORMATS}
               onChange={setSubtitleFormat}
             />
             <p className="text-xs text-zinc-500 dark:text-zinc-400">
-              SRT and VTT use the edited timeline (cuts applied). JSON keeps the
-              full word list for re-import.
+              {t("export.subtitlesHelp")}
             </p>
           </div>
         )}
 
         {error && (
           <p className="mb-4 rounded-lg bg-red-50 px-3 py-2 text-sm text-red-600 dark:border-red-900/30 dark:bg-red-950/30 dark:text-red-900">
-            {error}
+            {localizeRuntimeMessage(error, t)}
           </p>
         )}
 
@@ -417,7 +427,7 @@ export default function ExportDialog() {
             <div>
               <div className="mb-2 flex items-center justify-between text-sm">
                 <span className="font-medium text-zinc-700 dark:text-zinc-200">
-                  Rendering in your browser…
+                  {t("export.rendering")}
                 </span>
                 <span className="tabular-nums text-zinc-400 dark:text-zinc-500">
                   {Math.round(progress * 100)}%
@@ -430,7 +440,7 @@ export default function ExportDialog() {
                 />
               </div>
               <p className="mt-3 text-xs text-zinc-400 dark:text-zinc-500">
-                Re-encoding with ffmpeg.wasm — longer files take a while.
+                {t("export.encodingHelp")}
               </p>
             </div>
           ) : exportUrl ? (
@@ -441,13 +451,13 @@ export default function ExportDialog() {
                 className="flex h-10 items-center justify-center gap-2 rounded-xl bg-neutral-600 px-4 text-sm font-medium text-white transition hover:bg-neutral-500 dark:bg-neutral-500 dark:hover:bg-neutral-400"
               >
                 <Download size={15} className="shrink-0" />
-                <span className="truncate">Download {mediaFileName}</span>
+                <span className="truncate">{t("export.downloadFile", { name: mediaFileName })}</span>
               </a>
               <button
                 onClick={startMediaExport}
                 className="h-10 rounded-xl text-sm font-medium text-zinc-500 transition hover:bg-zinc-50 dark:text-zinc-400 dark:hover:bg-zinc-800"
               >
-                Re-export with latest edits
+                {t("export.reexport")}
               </button>
             </div>
           ) : (
@@ -456,10 +466,12 @@ export default function ExportDialog() {
               className="flex h-10 w-full items-center justify-center gap-2 rounded-xl bg-zinc-900 text-sm font-medium text-white transition hover:bg-zinc-700 dark:bg-zinc-100 dark:text-zinc-900 dark:hover:bg-zinc-200"
             >
               <Download size={15} />
-              Export{" "}
-              {activeTab === "audio"
-                ? audioFormat.toUpperCase()
-                : videoFormat.toUpperCase()}
+              {t("export.exportFormat", {
+                format:
+                  activeTab === "audio"
+                    ? audioFormat.toUpperCase()
+                    : videoFormat.toUpperCase(),
+              })}
             </button>
           ))}
 
@@ -470,7 +482,7 @@ export default function ExportDialog() {
             className="flex h-10 w-full items-center justify-center gap-2 rounded-xl bg-zinc-900 text-sm font-medium text-white transition hover:bg-zinc-700 disabled:opacity-40 dark:bg-zinc-100 dark:text-zinc-900 dark:hover:bg-zinc-200"
           >
             <Download size={15} />
-            Download .{transcriptFormat}
+            {t("export.downloadFormat", { format: transcriptFormat })}
           </button>
         )}
 
@@ -481,7 +493,7 @@ export default function ExportDialog() {
             className="flex h-10 w-full items-center justify-center gap-2 rounded-xl bg-zinc-900 text-sm font-medium text-white transition hover:bg-zinc-700 disabled:opacity-40 dark:bg-zinc-100 dark:text-zinc-900 dark:hover:bg-zinc-200"
           >
             <Download size={15} />
-            Download .{subtitleFormat}
+            {t("export.downloadFormat", { format: subtitleFormat })}
           </button>
         )}
       </div>
diff --git a/components/I18nProvider.tsx b/components/I18nProvider.tsx
new file mode 100644
index 0000000..4b73359
--- /dev/null
+++ b/components/I18nProvider.tsx
@@ -0,0 +1,84 @@
+"use client";
+
+import {
+  createContext,
+  useCallback,
+  useContext,
+  useEffect,
+  useMemo,
+  useState,
+  type ReactNode,
+} from "react";
+import {
+  loadUiLocalePreference,
+  resolveUiLocale,
+  saveUiLocalePreference,
+  systemLanguages,
+  translate,
+  UI_LOCALE_STORAGE_KEY,
+  type MessageKey,
+  type Translate,
+  type UiLocale,
+  type UiLocalePreference,
+} from "@/lib/i18n";
+
+interface I18nContextValue {
+  locale: UiLocale;
+  preference: UiLocalePreference;
+  setPreference: (preference: UiLocalePreference) => void;
+  t: Translate;
+}
+
+const I18nContext = createContext<I18nContextValue | null>(null);
+
+export default function I18nProvider({ children }: { children: ReactNode }) {
+  const [preference, setPreferenceState] = useState<UiLocalePreference>(
+    loadUiLocalePreference
+  );
+  const [languages, setLanguages] = useState<string[]>(systemLanguages);
+  const locale = resolveUiLocale(preference, languages);
+
+  const setPreference = useCallback((next: UiLocalePreference) => {
+    saveUiLocalePreference(next);
+    setPreferenceState(next);
+  }, []);
+
+  useEffect(() => {
+    const refreshSystemLanguages = () => setLanguages(systemLanguages());
+    const syncStorage = (event: StorageEvent) => {
+      if (event.key !== UI_LOCALE_STORAGE_KEY) return;
+      setPreferenceState(loadUiLocalePreference());
+    };
+    window.addEventListener("languagechange", refreshSystemLanguages);
+    window.addEventListener("storage", syncStorage);
+    return () => {
+      window.removeEventListener("languagechange", refreshSystemLanguages);
+      window.removeEventListener("storage", syncStorage);
+    };
+  }, []);
+
+  useEffect(() => {
+    document.documentElement.lang = locale;
+    document.title = translate(locale, "app.title");
+    window.rescriptDesktop?.setUiLocale(locale);
+  }, [locale]);
+
+  const t = useCallback(
+    (key: MessageKey, params?: Record<string, string | number>) =>
+      translate(locale, key, params),
+    [locale]
+  );
+
+  const value = useMemo<I18nContextValue>(
+    () => ({ locale, preference, setPreference, t }),
+    [locale, preference, setPreference, t]
+  );
+
+  return <I18nContext.Provider value={value}>{children}</I18nContext.Provider>;
+}
+
+export function useI18n(): I18nContextValue {
+  const value = useContext(I18nContext);
+  if (!value) throw new Error("useI18n must be used inside I18nProvider");
+  return value;
+}
diff --git a/components/ImportTranscriptOption.tsx b/components/ImportTranscriptOption.tsx
index 1b7b806..6d94aaa 100644
--- a/components/ImportTranscriptOption.tsx
+++ b/components/ImportTranscriptOption.tsx
@@ -5,7 +5,6 @@ import { FileText, Loader2 } from "lucide-react";
 import {
   isTranscriptFile,
   parseTranscriptFile,
-  TRANSCRIPT_FILE_ERROR,
   TRANSCRIPT_ACCEPT,
 } from "@/lib/parseTranscript";
 import { isModelId, type ModelId } from "@/lib/models";
@@ -16,6 +15,8 @@ import {
   useOptionTrigger,
   type ModelOptionContextValue,
 } from "./ModelSelector";
+import { useI18n } from "./I18nProvider";
+import { localizeRuntimeMessage } from "@/lib/i18n";
 
 /**
  * ModelSelector option that opens a caption file picker and surfaces parse
@@ -28,6 +29,7 @@ import {
  * menu the user just reopened.
  */
 export default function ImportTranscriptOption() {
+  const { t } = useI18n();
   const pendingTranscript = useEditorStore((s) => s.pendingTranscript);
   const setPendingTranscript = useEditorStore((s) => s.setPendingTranscript);
   const setSource = useEditorStore((s) => s.setSource);
@@ -94,12 +96,12 @@ export default function ImportTranscriptOption() {
   }, [picking, finishCancel]);
 
   const triggerLabel = reading
-    ? "Import…"
+    ? t("import.importing")
     : error
-      ? "Import failed"
+      ? t("import.failed")
       : pendingTranscript
         ? pendingTranscript.name
-        : "Import transcript";
+        : t("model.importTranscript");
 
   // Keep the custom trigger registered whenever import owns (or is about to
   // own) the closed button — otherwise ModelSelector falls back to the raw id
@@ -129,7 +131,7 @@ export default function ImportTranscriptOption() {
             }
             if (!isTranscriptFile(file)) {
               setPicking(false);
-              setError(TRANSCRIPT_FILE_ERROR);
+              setError(t("transcript.invalidFile"));
               setPendingTranscript(null);
               setSource("import");
               menu?.keepMenuOpen();
@@ -155,8 +157,8 @@ export default function ImportTranscriptOption() {
               setPendingTranscript(null);
               setError(
                 err instanceof Error
-                  ? err.message
-                  : "Could not read that transcript."
+                  ? localizeRuntimeMessage(err.message, t)
+                  : t("error.readTranscript")
               );
               setSource("import");
               menu?.keepMenuOpen();
@@ -168,7 +170,7 @@ export default function ImportTranscriptOption() {
       />
       <ModelOption
         id="import"
-        label="Import transcript"
+        label={t("model.importTranscript")}
         meta="SRT / VTT / JSON"
         icon={FileText}
         autoTrigger={false}
@@ -240,22 +242,23 @@ function ImportStatus({
   picking: boolean;
 }) {
   const { selected } = useModelOption();
+  const { t } = useI18n();
   if (reading) {
     return (
       <p className="mt-0.5 text-[11px] text-zinc-500 dark:text-zinc-400">
-        Reading file…
+        {t("import.readingFile")}
       </p>
     );
   }
   if (error) {
     return (
-      <p className="mt-0.5 text-[11px] text-red-600 dark:text-red-400">{error}</p>
+      <p className="mt-0.5 text-[11px] text-red-600 dark:text-red-400">{localizeRuntimeMessage(error, t)}</p>
     );
   }
   if (picking && !selected) {
     return (
       <p className="mt-0.5 text-[11px] text-zinc-500 dark:text-zinc-400">
-        Choose a file…
+        {t("import.chooseFileShort")}
       </p>
     );
   }
diff --git a/components/LogoLoader.tsx b/components/LogoLoader.tsx
index 57c77b2..170566f 100644
--- a/components/LogoLoader.tsx
+++ b/components/LogoLoader.tsx
@@ -1,6 +1,7 @@
 "use client";
 
 import { useId } from "react";
+import { useI18n } from "./I18nProvider";
 
 /** Filled silhouette of the Rescript "R", used to clip the sliding bars. */
 const R_PATH =
@@ -33,6 +34,7 @@ export default function LogoLoader({
   size?: number;
   className?: string;
 }) {
+  const { t } = useI18n();
   // Ids must be unique per instance, and React's contain colons that trip up url().
   const clipId = `logo-r-${useId().replace(/:/g, "")}`;
   const markHeight = size * 0.56;
@@ -40,7 +42,7 @@ export default function LogoLoader({
   return (
     <div
       role="img"
-      aria-label="Loading"
+      aria-label={t("common.loading")}
       className={`flex items-center justify-center rounded-[22%] bg-transparent text-zinc-900 dark:text-zinc-100 ${className}`}
       style={{ width: size, height: size }}
     >
diff --git a/components/ModelSelector.tsx b/components/ModelSelector.tsx
index 6b56c91..196516f 100644
--- a/components/ModelSelector.tsx
+++ b/components/ModelSelector.tsx
@@ -26,9 +26,10 @@ import {
   SignalBarsMedium,
 } from "./SignalBars";
 import {
-  TRANSCRIPT_LANGUAGE_ORDER,
+  AUTO_TRANSCRIPT_LANGUAGE_INFO,
+  TRANSCRIPT_LANGUAGE_PREFERENCE_ORDER,
   TRANSCRIPT_LANGUAGES,
-  type TranscriptLanguage,
+  type TranscriptLanguagePreference,
 } from "@/lib/languages";
 import {
   MODEL_ORDER,
@@ -43,6 +44,7 @@ import {
   useEditorStore,
 } from "@/lib/store";
 import Popover, { PopoverContent, PopoverTrigger } from "./Popover";
+import { useI18n } from "./I18nProvider";
 
 export type ModelOptionContextValue = {
   /** Currently selected source id. */
@@ -140,6 +142,7 @@ export default function ModelSelector({
   const source = useEditorStore((s) => s.source);
   const setSource = useEditorStore((s) => s.setSource);
   const transcriptLanguage = useEditorStore((s) => s.transcriptLanguage);
+  const { t } = useI18n();
   const [open, setOpen] = useState(false);
   const [triggers, setTriggers] = useState<Record<string, OptionTrigger>>({});
   const listId = useId();
@@ -205,13 +208,15 @@ export default function ModelSelector({
     (isModelId(source)
       ? MODELS[source].label
       : source === "import"
-        ? "Import transcript"
+        ? t("model.importTranscript")
         : String(source));
-  const languageInfo = TRANSCRIPT_LANGUAGES[transcriptLanguage];
+  const languageInfo =
+    transcriptLanguage === "auto"
+      ? AUTO_TRANSCRIPT_LANGUAGE_INFO
+      : TRANSCRIPT_LANGUAGES[transcriptLanguage];
   const showLanguageInTrigger =
     isWhisperModel(source) &&
-    !activeTrigger?.busy &&
-    transcriptLanguage !== "en";
+    !activeTrigger?.busy;
 
   // Always mount options (hidden when closed) so custom triggers stay registered.
   const options = children ?? (
@@ -252,7 +257,7 @@ export default function ModelSelector({
               aria-controls={listId}
               aria-label={
                 showLanguageInTrigger
-                  ? `${typeof baseTriggerLabel === "string" ? baseTriggerLabel : "Transcript source"}, ${languageInfo.label}`
+                  ? `${typeof baseTriggerLabel === "string" ? baseTriggerLabel : t("model.transcriptSource")}, ${languageInfo.label}`
                   : undefined
               }
               onClick={() => setOpen((v) => !v)}
@@ -416,9 +421,13 @@ export function LanguageSection() {
   const selector = useSelectorCtx();
   const [submenuOpen, setSubmenuOpen] = useState(false);
   const submenuId = useId();
-  const active = TRANSCRIPT_LANGUAGES[language];
+  const { t } = useI18n();
+  const active =
+    language === "auto"
+      ? AUTO_TRANSCRIPT_LANGUAGE_INFO
+      : TRANSCRIPT_LANGUAGES[language];
 
-  const select = (next: TranscriptLanguage) => {
+  const select = (next: TranscriptLanguagePreference) => {
     setLanguage(next);
     setSubmenuOpen(false);
     selector.closeMenu();
@@ -427,7 +436,7 @@ export function LanguageSection() {
   return (
     <div>
       <p className="px-2.5 pb-1 pt-1.5 text-[11px] font-medium tracking-wide text-zinc-400 dark:text-zinc-500">
-        Language
+        {t("model.language")}
       </p>
       {/* No portal: stay in the parent panel DOM so outside-click on the model
           menu still treats this flyout as inside the floating tree. */}
@@ -459,7 +468,7 @@ export function LanguageSection() {
                 {active.flag}
               </span>
               <span className="min-w-0 flex-1 text-[13px] font-medium leading-tight">
-                {active.nativeLabel}
+                {language === "auto" ? t("common.auto") : active.nativeLabel}
               </span>
               <ChevronRight
                 size={14}
@@ -472,11 +481,14 @@ export function LanguageSection() {
           <PopoverContent
             id={submenuId}
             role="menu"
-            aria-label="Transcript language"
+            aria-label={t("model.transcriptLanguage")}
             className="z-50 w-44 overflow-hidden p-1"
           >
-            {TRANSCRIPT_LANGUAGE_ORDER.map((id) => {
-              const option = TRANSCRIPT_LANGUAGES[id];
+            {TRANSCRIPT_LANGUAGE_PREFERENCE_ORDER.map((id) => {
+              const option =
+                id === "auto"
+                  ? AUTO_TRANSCRIPT_LANGUAGE_INFO
+                  : TRANSCRIPT_LANGUAGES[id];
               const selected = id === language;
               return (
                 <button
@@ -494,7 +506,7 @@ export function LanguageSection() {
                     {option.flag}
                   </span>
                   <span className="min-w-0 flex-1 text-[13px] font-medium leading-tight">
-                    {option.nativeLabel}
+                    {id === "auto" ? t("common.auto") : option.nativeLabel}
                   </span>
                   {selected && (
                     <Check
@@ -508,6 +520,11 @@ export function LanguageSection() {
           </PopoverContent>
         </div>
       </Popover>
+      {selector.value === "parakeet" && (
+        <p className="px-2.5 pb-1 pt-1 text-[11px] leading-relaxed text-zinc-400 dark:text-zinc-500">
+          {t("model.parakeetAuto")}
+        </p>
+      )}
     </div>
   );
 }
diff --git a/components/SettingsMenu.tsx b/components/SettingsMenu.tsx
index fc00f33..309950f 100644
--- a/components/SettingsMenu.tsx
+++ b/components/SettingsMenu.tsx
@@ -15,17 +15,25 @@ import { useAppearance } from "@/hooks/useAppearance";
 import { useTelemetryPref } from "@/hooks/useTelemetryPref";
 import Popover, { PopoverContent, PopoverTrigger } from "./Popover";
 import type { Appearance } from "@/lib/theme";
+import { useI18n } from "./I18nProvider";
+import { useEditorStore } from "@/lib/store";
+import {
+  TRANSCRIPT_LANGUAGE_PREFERENCE_ORDER,
+  TRANSCRIPT_LANGUAGES,
+  type TranscriptLanguagePreference,
+} from "@/lib/languages";
+import type { UiLocalePreference } from "@/lib/i18n";
 
 const MENU_LINKS = [
-  { label: "Support / feedback", href: DISCORD_INVITE_URL, Icon: DiscordIcon },
+  { labelKey: "settings.support", href: DISCORD_INVITE_URL, Icon: DiscordIcon },
   {
-    label: "Report an issue",
+    labelKey: "settings.reportIssue",
     href: `${GITHUB_REPO_URL}/issues`,
     Icon: Bug,
   },
-  { label: "Homepage", href: WEBSITE_URL, Icon: HomeIcon },
-  { label: "Github", href: GITHUB_REPO_URL, Icon: GitHubIcon },
-  { label: "Follow on X", href: X_PROFILE_URL, Icon: XIcon },
+  { labelKey: "settings.homepage", href: WEBSITE_URL, Icon: HomeIcon },
+  { labelKey: "settings.github", href: GITHUB_REPO_URL, Icon: GitHubIcon },
+  { labelKey: "settings.followX", href: X_PROFILE_URL, Icon: XIcon },
 ] as const;
 
 /**
@@ -37,6 +45,9 @@ export default function SettingsMenu() {
   const panelId = useId();
   const { appearance, setAppearance } = useAppearance();
   const { enabled: telemetry, setEnabled: setTelemetry } = useTelemetryPref();
+  const { t, preference, setPreference } = useI18n();
+  const transcriptLanguage = useEditorStore((s) => s.transcriptLanguage);
+  const setTranscriptLanguage = useEditorStore((s) => s.setTranscriptLanguage);
 
   return (
     <Popover
@@ -49,11 +60,11 @@ export default function SettingsMenu() {
         <PopoverTrigger>
           <button
             type="button"
-            aria-label="Settings"
+            aria-label={t("common.settings")}
             aria-haspopup="dialog"
             aria-expanded={open}
             aria-controls={panelId}
-            title="Settings"
+            title={t("common.settings")}
             onClick={() => setOpen((v) => !v)}
             className="flex h-8 w-8 cursor-pointer items-center justify-center rounded-lg text-zinc-500 transition hover:bg-zinc-100 hover:text-zinc-800 dark:text-zinc-400 dark:hover:bg-zinc-800 dark:hover:text-zinc-200"
           >
@@ -64,28 +75,28 @@ export default function SettingsMenu() {
         <PopoverContent
           id={panelId}
           role="dialog"
-          aria-label="Settings"
+          aria-label={t("common.settings")}
           className="z-40 w-[15rem] overflow-hidden"
         >
           <section className="border-b border-zinc-100 px-3 py-2.5 dark:border-zinc-800">
             <p className="mb-2 text-[11px] font-medium tracking-wide text-zinc-400 dark:text-zinc-500">
-              Appearance
+              {t("settings.appearance")}
             </p>
             <div
               className="grid grid-cols-2 gap-0.5 rounded-lg bg-zinc-100 p-0.5 dark:bg-zinc-800"
               role="radiogroup"
-              aria-label="Appearance"
+              aria-label={t("settings.appearance")}
             >
               <AppearanceOption
                 value="light"
-                label="Light"
+                label={t("settings.light")}
                 icon={Sun}
                 selected={appearance === "light"}
                 onSelect={setAppearance}
               />
               <AppearanceOption
                 value="dark"
-                label="Dark"
+                label={t("settings.dark")}
                 icon={Moon}
                 selected={appearance === "dark"}
                 onSelect={setAppearance}
@@ -93,10 +104,53 @@ export default function SettingsMenu() {
             </div>
           </section>
 
+          <section className="border-b border-zinc-100 px-3 py-2.5 dark:border-zinc-800">
+            <label className="block text-[11px] font-medium tracking-wide text-zinc-400 dark:text-zinc-500">
+              {t("settings.interfaceLanguage")}
+              <select
+                value={preference}
+                onChange={(event) =>
+                  setPreference(event.target.value as UiLocalePreference)
+                }
+                className="mt-2 block h-8 w-full rounded-lg border border-zinc-200 bg-white px-2 text-[12px] text-zinc-700 outline-none dark:border-zinc-700 dark:bg-zinc-900 dark:text-zinc-200"
+              >
+                <option value="system">{t("common.system")}</option>
+                <option value="en">{t("language.english")}</option>
+                <option value="zh-CN">{t("language.simplifiedChinese")}</option>
+              </select>
+            </label>
+          </section>
+
+          <section className="border-b border-zinc-100 px-3 py-2.5 dark:border-zinc-800">
+            <label className="block text-[11px] font-medium tracking-wide text-zinc-400 dark:text-zinc-500">
+              {t("settings.transcriptLanguage")}
+              <select
+                value={transcriptLanguage}
+                onChange={(event) =>
+                  setTranscriptLanguage(
+                    event.target.value as TranscriptLanguagePreference
+                  )
+                }
+                className="mt-2 block h-8 w-full rounded-lg border border-zinc-200 bg-white px-2 text-[12px] text-zinc-700 outline-none dark:border-zinc-700 dark:bg-zinc-900 dark:text-zinc-200"
+              >
+                {TRANSCRIPT_LANGUAGE_PREFERENCE_ORDER.map((id) => (
+                  <option key={id} value={id}>
+                    {id === "auto"
+                      ? t("common.auto")
+                      : TRANSCRIPT_LANGUAGES[id].nativeLabel}
+                  </option>
+                ))}
+              </select>
+            </label>
+            <p className="mt-1.5 text-[10px] leading-snug text-zinc-400 dark:text-zinc-500">
+              {t("settings.transcriptLanguageHelp")}
+            </p>
+          </section>
+
           <section className="border-b border-zinc-100 px-1.5 py-1.5 dark:border-zinc-800">
-            {MENU_LINKS.map(({ label, href, Icon }) => (
+            {MENU_LINKS.map(({ labelKey, href, Icon }) => (
               <a
-                key={label}
+                key={labelKey}
                 href={href}
                 target="_blank"
                 rel="noopener noreferrer"
@@ -109,7 +163,7 @@ export default function SettingsMenu() {
                 <span className="shrink-0 text-zinc-400 dark:text-zinc-500">
                   <Icon size={14} />
                 </span>
-                <span className="flex-1">{label}</span>
+                <span className="flex-1">{t(labelKey)}</span>
                 <ExternalLink
                   size={12}
                   className="shrink-0 text-zinc-300 dark:text-zinc-600"
@@ -120,7 +174,7 @@ export default function SettingsMenu() {
 
           <section className="px-2 py-2.5">
             <p className="mb-2 text-[11px] font-medium tracking-wide text-zinc-400 dark:text-zinc-500">
-              Privacy
+              {t("settings.privacy")}
             </p>
             <label className="flex cursor-pointer items-start gap-2.5">
               <input
@@ -131,10 +185,10 @@ export default function SettingsMenu() {
               />
               <span>
                 <span className="block text-[12px] text-zinc-700 dark:text-zinc-300">
-                  Help improve the app
+                  {t("settings.helpImprove")}
                 </span>
                 <span className="mt-0.5 block text-[11px] leading-snug text-zinc-400 dark:text-zinc-500">
-                  Send anonymous feature usage statistics and crash reports.
+                  {t("settings.telemetryHelp")}
                 </span>
               </span>
             </label>
diff --git a/components/SocialLinks.tsx b/components/SocialLinks.tsx
index 57dd470..47931a2 100644
--- a/components/SocialLinks.tsx
+++ b/components/SocialLinks.tsx
@@ -2,6 +2,7 @@ export const GITHUB_REPO_URL = "https://github.com/wassgha/rescript";
 export const DISCORD_INVITE_URL = "https://discord.gg/qJAhYFydat";
 export const X_PROFILE_URL = "https://x.com/wassgha";
 export const WEBSITE_URL = "https://getrescript.com";
+import { useI18n } from "./I18nProvider";
 
 export function GitHubIcon({ size }: { size: number }) {
   return (
@@ -49,19 +50,19 @@ const LINKS = [
   {
     label: "GitHub",
     href: GITHUB_REPO_URL,
-    ariaLabel: "GitHub repository",
+    ariaKey: "social.githubRepo" as const,
     Icon: GitHubIcon,
   },
   {
     label: "Discord",
     href: DISCORD_INVITE_URL,
-    ariaLabel: "Discord server",
+    ariaKey: "social.discordServer" as const,
     Icon: DiscordIcon,
   },
   {
     label: "X",
     href: X_PROFILE_URL,
-    ariaLabel: "X profile",
+    ariaKey: "social.xProfile" as const,
     Icon: XIcon,
   },
 ];
@@ -72,6 +73,7 @@ export default function SocialLinks({
 }: {
   variant?: "icon" | "text";
 }) {
+  const { t } = useI18n();
   if (variant === "text") {
     return (
       <div className="flex items-center gap-3">
@@ -93,14 +95,14 @@ export default function SocialLinks({
 
   return (
     <>
-      {LINKS.map(({ label, href, ariaLabel, Icon }) => (
+      {LINKS.map(({ label, href, ariaKey, Icon }) => (
         <a
           key={label}
           href={href}
           target="_blank"
           rel="noopener noreferrer"
           title={label}
-          aria-label={ariaLabel}
+          aria-label={t(ariaKey)}
           className="flex h-8 w-8 items-center justify-center rounded-lg text-zinc-300 transition hover:bg-zinc-100 hover:text-zinc-800 dark:text-zinc-600 dark:hover:bg-zinc-800 dark:hover:text-zinc-200"
         >
           <Icon size={16} />
diff --git a/components/SpeakerLabel.tsx b/components/SpeakerLabel.tsx
index 08a5697..df370d6 100644
--- a/components/SpeakerLabel.tsx
+++ b/components/SpeakerLabel.tsx
@@ -30,9 +30,16 @@ import {
 import type { SpeakerInfo } from "@/lib/types";
 import { useWordAnchorFloating } from "@/hooks/useWordAnchorFloating";
 import Popover, { PopoverContent, PopoverTrigger } from "./Popover";
+import { useI18n } from "./I18nProvider";
+import type { Translate } from "@/lib/i18n";
 
 type PickerMode = "change" | "rename" | "replace";
 
+function localizedSpeakerName(name: string, t: Translate): string {
+  const match = /^Speaker (\d+)$/.exec(name);
+  return match ? t("speaker.defaultName", { number: match[1] }) : name;
+}
+
 /**
  * Speaker heading: rename / change / replace / remove, plus a grip to drag
  * the turn boundary onto another word in the adjacent turns.
@@ -50,6 +57,7 @@ export default function SpeakerLabel({
   /** False for the first turn — nothing to merge a boundary against. */
   canMove: boolean;
 }) {
+  const { t } = useI18n();
   const speakers = useEditorStore((s) => s.speakers);
   const changeTurnSpeaker = useEditorStore((s) => s.changeTurnSpeaker);
   const renameSpeaker = useEditorStore((s) => s.renameSpeaker);
@@ -60,6 +68,7 @@ export default function SpeakerLabel({
   const moveSpeakerLabel = useEditorStore((s) => s.moveSpeakerLabel);
 
   const label = speakerLabel(speakers, speakerId);
+  const displayLabel = localizedSpeakerName(label, t);
   const color = speakerColor(speakerId);
 
   const [pickerOpen, setPickerOpen] = useState(false);
@@ -209,18 +218,18 @@ export default function SpeakerLabel({
 
   const title =
     mode === "rename"
-      ? "Rename speaker"
+      ? t("speaker.rename")
       : mode === "replace"
-        ? "Replace in project with…"
-        : "Change speaker";
+        ? t("speaker.replace")
+        : t("speaker.change");
 
   return (
     <div className="mb-1.5 flex items-center gap-0.5">
       {canMove && (
         <button
           type="button"
-          title="Drag to move where this speaker starts"
-          aria-label="Move speaker label"
+          title={t("speaker.moveStart")}
+          aria-label={t("speaker.moveLabel")}
           onPointerDown={onGripPointerDown}
           className="flex h-6 w-5 cursor-grab items-center justify-center rounded text-zinc-300 transition hover:bg-zinc-100 hover:text-zinc-500 active:cursor-grabbing dark:text-zinc-600 dark:hover:bg-zinc-800 dark:hover:text-zinc-300"
         >
@@ -240,7 +249,7 @@ export default function SpeakerLabel({
               aria-expanded={pickerOpen}
               aria-controls={panelId}
             >
-              {label}
+              {displayLabel}
               <ChevronDown
                 size={12}
                 className="opacity-50 transition group-hover:opacity-80"
@@ -270,10 +279,10 @@ export default function SpeakerLabel({
                 }}
                 placeholder={
                   mode === "rename"
-                    ? "New name"
+                    ? t("speaker.newName")
                     : mode === "replace"
-                      ? "Find a speaker…"
-                      : "Search or create…"
+                      ? t("speaker.find")
+                      : t("speaker.search")
                 }
                 className="w-full rounded-lg border border-zinc-200 bg-zinc-50 px-2 py-1.5 text-[13px] text-zinc-800 outline-none focus:border-zinc-400 dark:border-zinc-700 dark:bg-zinc-950 dark:text-zinc-100 dark:focus:border-zinc-500"
               />
@@ -288,8 +297,10 @@ export default function SpeakerLabel({
                 >
                   <UserRoundPen size={13} className="shrink-0 text-zinc-400" />
                   <span>
-                    Rename <span className="font-medium">{label}</span> to{" "}
-                    <span className="font-medium">{trimmedQuery}</span>
+                    {t("speaker.renameTo", {
+                      current: displayLabel,
+                      next: trimmedQuery,
+                    })}
                   </span>
                 </button>
               )}
@@ -301,13 +312,13 @@ export default function SpeakerLabel({
                 >
                   <Plus size={13} className="shrink-0 text-zinc-400" />
                   <span>
-                    Create <span className="font-medium">“{trimmedQuery}”</span>
+                    {t("speaker.create", { name: trimmedQuery })}
                   </span>
                 </button>
               )}
               {filtered.length === 0 && !canCreate && !canRenameTo && (
                 <p className="px-2.5 py-2 text-[12px] text-zinc-400">
-                  No matching speakers
+                  {t("speaker.noMatches")}
                 </p>
               )}
               {filtered.map((s) => (
@@ -329,8 +340,8 @@ export default function SpeakerLabel({
           <PopoverTrigger>
             <button
               type="button"
-              title="Speaker options"
-              aria-label="Speaker options"
+              title={t("speaker.options")}
+              aria-label={t("speaker.options")}
               aria-haspopup="menu"
               aria-expanded={menuOpen}
               onClick={() => setMenuOpen((v) => !v)}
@@ -341,23 +352,23 @@ export default function SpeakerLabel({
           </PopoverTrigger>
           <PopoverContent
             role="menu"
-            aria-label="Speaker options"
+            aria-label={t("speaker.options")}
             className="z-40 w-56 overflow-hidden py-1"
           >
             <MenuItem
               icon={UserRoundPen}
-              label="Rename"
+              label={t("speaker.renameAction")}
               onClick={() => openPicker("rename")}
             />
             <MenuItem
               icon={Replace}
-              label="Replace in project with…"
+              label={t("speaker.replace")}
               disabled={speakers.length < 2}
               onClick={() => openPicker("replace")}
             />
             <MenuItem
               icon={Trash2}
-              label="Remove from project"
+              label={t("speaker.removeProject")}
               disabled={speakers.length < 2}
               danger
               onClick={() => {
@@ -374,6 +385,7 @@ export default function SpeakerLabel({
 
 /** Toolbar button that opens the selection speaker picker. */
 export function SelectionSpeakerButton({ onClick }: { onClick: () => void }) {
+  const { t } = useI18n();
   return (
     <button
       type="button"
@@ -382,7 +394,7 @@ export function SelectionSpeakerButton({ onClick }: { onClick: () => void }) {
       className="flex cursor-pointer items-center gap-1.5 rounded-lg px-2.5 py-1.5 text-[13px] font-medium text-zinc-700 transition hover:bg-zinc-100 dark:text-zinc-200 dark:hover:bg-zinc-700"
     >
       <UserRoundPen size={13} />
-      Speaker
+      {t("speaker.button")}
     </button>
   );
 }
@@ -397,6 +409,7 @@ export function SelectionSpeakerPopover({
   containerRef: RefObject<HTMLElement | null>;
   onClose: () => void;
 }) {
+  const { t } = useI18n();
   const speakers = useEditorStore((s) => s.speakers);
   const words = useEditorStore((s) => s.words);
   const changeTurnSpeaker = useEditorStore((s) => s.changeTurnSpeaker);
@@ -467,7 +480,7 @@ export function SelectionSpeakerPopover({
         className="z-40 w-60 max-w-[calc(100vw-16px)] overflow-hidden rounded-2xl border border-zinc-200 bg-white shadow-xl shadow-zinc-900/10 dark:border-zinc-700 dark:bg-zinc-800 dark:shadow-black/40"
         style={floatingStyles}
         role="dialog"
-        aria-label="Change speaker"
+        aria-label={t("speaker.change")}
       >
         <div className="border-b border-zinc-100 px-2.5 py-2 dark:border-zinc-700">
           <input
@@ -480,7 +493,7 @@ export function SelectionSpeakerPopover({
               if (exact) apply(exact.id);
               else if (canCreate) apply("new", trimmedQuery);
             }}
-            placeholder="Search or create…"
+            placeholder={t("speaker.search")}
             className="w-full rounded-lg border border-zinc-200 bg-zinc-50 px-2 py-1.5 text-[13px] text-zinc-800 outline-none focus:border-zinc-400 dark:border-zinc-600 dark:bg-zinc-900 dark:text-zinc-100 dark:focus:bg-zinc-900 dark:focus:border-zinc-400"
           />
         </div>
@@ -493,7 +506,7 @@ export function SelectionSpeakerPopover({
             >
               <Plus size={13} className="shrink-0 text-zinc-400" />
               <span>
-                Create <span className="font-medium">“{trimmedQuery}”</span>
+                {t("speaker.create", { name: trimmedQuery })}
               </span>
             </button>
           )}
@@ -522,6 +535,7 @@ function SpeakerOption({
   disabled?: boolean;
   onSelect: () => void;
 }) {
+  const { t } = useI18n();
   return (
     <button
       type="button"
@@ -533,7 +547,7 @@ function SpeakerOption({
         className="h-2 w-2 shrink-0 rounded-full"
         style={{ backgroundColor: speakerColor(speaker.id) }}
       />
-      <span className="flex-1 truncate font-medium">{speaker.name}</span>
+      <span className="flex-1 truncate font-medium">{localizedSpeakerName(speaker.name, t)}</span>
       {active && <Check size={13} className="shrink-0 text-zinc-400" />}
     </button>
   );
diff --git a/components/Timeline.tsx b/components/Timeline.tsx
index 1bc5575..6c2c320 100644
--- a/components/Timeline.tsx
+++ b/components/Timeline.tsx
@@ -41,6 +41,7 @@ import { VAD_SAMPLE_RATE } from "@/lib/vad";
 import { peakBetween } from "@/lib/waveform";
 import { useCutRanges } from "@/hooks/useCutRanges";
 import { useIsDark } from "@/hooks/useIsDark";
+import { useI18n } from "./I18nProvider";
 
 const RULER_H = 18;
 const WORDBAR_H = 28;
@@ -95,6 +96,7 @@ type DragKind =
   | { type: "trim"; edge: "in" | "out"; time: number; lo: number; hi: number };
 
 export default function Timeline() {
+  const { t } = useI18n();
   const waveform = useEditorStore((s) => s.waveform);
   const words = useEditorStore((s) => s.words);
   const sceneBoundaries = useEditorStore((s) => s.sceneBoundaries);
@@ -614,7 +616,10 @@ export default function Timeline() {
           </span>
           {trimmed > 0.01 && (
             <span
-              title={`${formatTime(trimmed)} removed — original length ${formatTime(duration)}`}
+              title={t("timeline.removed", {
+                trimmed: formatTime(trimmed),
+                duration: formatTime(duration),
+              })}
               className="hidden sm:inline-block shrink-0 font-mono rounded-sm bg-red-50 px-1.5 py-0.5 text-[10px] font-medium tabular-nums leading-none text-red-600 dark:bg-red-950/40 dark:text-red-400"
             >
               −{formatTime(trimmed)}
@@ -627,7 +632,7 @@ export default function Timeline() {
             type="button"
             disabled={!ready}
             onClick={() => skip(-5)}
-            title="Back 5 s"
+            title={t("timeline.back5")}
             className="flex h-7 w-7 cursor-pointer items-center justify-center rounded-lg text-zinc-500 transition hover:bg-zinc-100 hover:text-zinc-900 disabled:cursor-not-allowed disabled:text-zinc-300 disabled:hover:bg-transparent dark:text-zinc-400 dark:hover:bg-zinc-800 dark:hover:text-zinc-100 dark:disabled:text-zinc-600"
           >
             <ChevronLeft size={14} />
@@ -636,7 +641,7 @@ export default function Timeline() {
             type="button"
             disabled={!ready}
             onClick={togglePlay}
-            title="Play / pause (space)"
+            title={t("timeline.playPause")}
             className="flex h-7 w-7 shrink-0 cursor-pointer items-center justify-center rounded-lg text-zinc-900 transition hover:bg-zinc-100 active:scale-95 disabled:cursor-not-allowed disabled:text-zinc-300 disabled:hover:bg-transparent dark:text-zinc-100 dark:hover:bg-zinc-800 dark:disabled:text-zinc-600"
           >
             {playing ? (
@@ -649,7 +654,7 @@ export default function Timeline() {
             type="button"
             disabled={!ready}
             onClick={() => skip(5)}
-            title="Forward 5 s"
+            title={t("timeline.forward5")}
             className="flex h-7 w-7 cursor-pointer items-center justify-center rounded-lg text-zinc-500 transition hover:bg-zinc-100 hover:text-zinc-900 disabled:cursor-not-allowed disabled:text-zinc-300 disabled:hover:bg-transparent dark:text-zinc-400 dark:hover:bg-zinc-800 dark:hover:text-zinc-100 dark:disabled:text-zinc-600"
           >
             <ChevronRight size={14} />
@@ -665,8 +670,8 @@ export default function Timeline() {
             }}
             title={
               splitOk
-                ? "Split clip at playhead (S)"
-                : "Move the playhead onto a kept region to split"
+                ? t("timeline.splitTitle")
+                : t("timeline.splitDisabled")
             }
             className={`flex h-7 items-center gap-1.5 rounded-lg px-2 text-xs font-medium transition ${
               ready && splitOk
@@ -675,7 +680,7 @@ export default function Timeline() {
             }`}
           >
             <SquareSplitHorizontal size={13} />
-            <span className="hidden sm:inline">Split</span>
+            <span className="hidden sm:inline">{t("timeline.split")}</span>
             <kbd
               className={`hidden rounded px-1 py-px text-[10px] font-normal sm:inline ${
                 ready && splitOk
@@ -695,8 +700,8 @@ export default function Timeline() {
             }}
             title={
               deleteOk
-                ? "Delete selected clip (Delete)"
-                : "Select a clip on the timeline to delete"
+                ? t("timeline.deleteTitle")
+                : t("timeline.deleteDisabled")
             }
             className={`flex h-7 items-center gap-1.5 rounded-lg px-2 text-xs font-medium transition ${
               ready && deleteOk
@@ -705,7 +710,7 @@ export default function Timeline() {
             }`}
           >
             <Trash2 size={13} />
-            <span className="hidden sm:inline">Delete</span>
+            <span className="hidden sm:inline">{t("timeline.delete")}</span>
             <kbd
               className={`hidden rounded px-1 py-px text-[10px] font-normal sm:inline ${
                 ready && deleteOk
@@ -731,8 +736,8 @@ export default function Timeline() {
             }}
             title={
               restoreOk
-                ? "Restore selected cut (Delete)"
-                : "Select a cut or silence section on the timeline to restore"
+                ? t("timeline.restoreTitle")
+                : t("timeline.restoreDisabled")
             }
             className={`flex h-7 items-center gap-1.5 rounded-lg px-2 text-xs font-medium transition ${
               ready && restoreOk
@@ -741,7 +746,7 @@ export default function Timeline() {
             }`}
           >
             <RotateCcw size={13} />
-            <span className="hidden sm:inline">Restore</span>
+            <span className="hidden sm:inline">{t("timeline.restore")}</span>
             <kbd
               className={`hidden rounded px-1 py-px text-[10px] font-normal sm:inline ${
                 ready && restoreOk
@@ -760,7 +765,7 @@ export default function Timeline() {
               type="button"
               onClick={() => setZoom((z) => Math.max(MIN_ZOOM, z / 1.5))}
               disabled={zoom <= MIN_ZOOM}
-              title="Zoom out"
+              title={t("timeline.zoomOut")}
               className="flex h-7 w-7 cursor-pointer items-center justify-center rounded-lg text-zinc-500 transition hover:bg-zinc-100 hover:text-zinc-900 disabled:cursor-not-allowed disabled:text-zinc-300 disabled:hover:bg-transparent dark:text-zinc-400 dark:hover:bg-zinc-800 dark:hover:text-zinc-100 dark:disabled:text-zinc-600"
             >
               <ZoomOut size={14} />
@@ -769,7 +774,7 @@ export default function Timeline() {
               type="button"
               onClick={() => setZoom(1)}
               disabled={zoom === 1}
-              title="Fit to window"
+              title={t("timeline.fit")}
               className="flex h-7 w-7 cursor-pointer items-center justify-center rounded-lg text-zinc-500 transition hover:bg-zinc-100 hover:text-zinc-900 disabled:cursor-not-allowed disabled:text-zinc-300 disabled:hover:bg-transparent dark:text-zinc-400 dark:hover:bg-zinc-800 dark:hover:text-zinc-100 dark:disabled:text-zinc-600"
             >
               <Maximize2 size={13} />
@@ -778,7 +783,7 @@ export default function Timeline() {
               type="button"
               onClick={() => setZoom((z) => Math.min(MAX_ZOOM, z * 1.5))}
               disabled={zoom >= MAX_ZOOM}
-              title="Zoom in — drag word edges to refine timing"
+              title={t("timeline.zoomIn")}
               className="flex h-7 w-7 cursor-pointer items-center justify-center rounded-lg text-zinc-500 transition hover:bg-zinc-100 hover:text-zinc-900 disabled:cursor-not-allowed disabled:text-zinc-300 disabled:hover:bg-transparent dark:text-zinc-400 dark:hover:bg-zinc-800 dark:hover:text-zinc-100 dark:disabled:text-zinc-600"
             >
               <ZoomIn size={14} />
@@ -822,8 +827,8 @@ export default function Timeline() {
                     <button
                       type="button"
                       data-tl-interactive
-                      title="Join these clips (remove split)"
-                      aria-label="Join clips"
+                      title={t("timeline.joinClips")}
+                      aria-label={t("transcript.joinClips")}
                       onPointerDown={(e) => e.stopPropagation()}
                       onClick={(e) => joinAtSplit(e, b.id)}
                       className="pointer-events-auto flex h-4 w-4 cursor-pointer items-center justify-center rounded-full border border-zinc-300 bg-white text-zinc-500 shadow-sm transition hover:border-zinc-400 hover:bg-zinc-100 hover:text-zinc-800 dark:border-zinc-600 dark:bg-zinc-800 dark:text-zinc-400 dark:hover:border-zinc-500 dark:hover:bg-zinc-700 dark:hover:text-zinc-200"
@@ -869,7 +874,7 @@ export default function Timeline() {
                       bottom: 4,
                       opacity: selected ? 1 : 0.7,
                     }}
-                    title="Trim clip start"
+                    title={t("timeline.trimStart")}
                   >
                     <div
                       className={`h-full w-1 rounded-full transition-all duration-150 ${
@@ -889,7 +894,7 @@ export default function Timeline() {
                       bottom: 4,
                       opacity: selected ? 1 : 0.7,
                     }}
-                    title="Trim clip end"
+                    title={t("timeline.trimEnd")}
                   >
                     <div
                       className={`h-full w-1 rounded-full transition-all duration-150 ${
@@ -948,10 +953,10 @@ export default function Timeline() {
                   title={
                     placeholder
                       ? showHandles
-                        ? "… detected hesitation — drag edges to adjust, or Remove filler words to cut"
-                        : "… detected hesitation — cut with Remove filler words"
+                        ? t("timeline.hesitationAdjust")
+                        : t("timeline.hesitationCut")
                       : showHandles
-                        ? `${w.text} — drag edges to adjust timing`
+                        ? t("timeline.dragTiming", { word: w.text })
                         : w.text
                   }
                   onPointerEnter={() => setHoveredWordId(w.id)}
@@ -1049,7 +1054,7 @@ export default function Timeline() {
 
         {pps < SMALL_PPS && ready && (
           <div className="pointer-events-none absolute bottom-2 left-1/2 z-10 -translate-x-1/2 rounded-full bg-zinc-900/70 px-2.5 py-1 text-[10px] text-white/90 backdrop-blur-sm transition-opacity dark:bg-zinc-100/80 dark:text-zinc-900">
-            Scroll to zoom in/out
+            {t("timeline.scrollZoom")}
           </div>
         )}
       </div>
diff --git a/components/TopBar.tsx b/components/TopBar.tsx
index 6456e72..de6e2db 100644
--- a/components/TopBar.tsx
+++ b/components/TopBar.tsx
@@ -4,6 +4,7 @@ import { useEditorStore } from "@/lib/store";
 import Image from "next/image";
 import logo from "@/assets/logo.png";
 import { useWindowChrome } from "@/hooks/useWindowChrome";
+import { useI18n } from "./I18nProvider";
 
 function truncateMiddle(value: string, maxLength = 40): string {
   if (value.length <= maxLength) return value;
@@ -15,6 +16,7 @@ function truncateMiddle(value: string, maxLength = 40): string {
 }
 
 export default function TopBar({ children }: { children?: React.ReactNode }) {
+  const { t } = useI18n();
   const { draggable, trafficLights } = useWindowChrome();
   const videoFile = useEditorStore((s) => s.videoFile);
   const reset = useEditorStore((s) => s.reset);
@@ -27,7 +29,7 @@ export default function TopBar({ children }: { children?: React.ReactNode }) {
     >
       <button
         onClick={reset}
-        title="Start over"
+        title={t("topbar.startOver")}
         className="app-no-drag flex h-7 w-7 cursor-pointer items-center justify-center rounded-lg text-zinc-500 transition hover:bg-zinc-100 hover:text-zinc-800 dark:text-zinc-400 dark:hover:bg-zinc-800 dark:hover:text-zinc-200"
       >
         <Image
diff --git a/components/TranscriptPanel.tsx b/components/TranscriptPanel.tsx
index 9534161..dfcbe2f 100644
--- a/components/TranscriptPanel.tsx
+++ b/components/TranscriptPanel.tsx
@@ -22,7 +22,6 @@ import TranscriptToolsMenu from "./TranscriptToolsMenu";
 import {
   isTranscriptFile,
   parseTranscriptFile,
-  TRANSCRIPT_FILE_ERROR,
   TRANSCRIPT_ACCEPT,
 } from "@/lib/parseTranscript";
 import type { Word } from "@/lib/types";
@@ -43,6 +42,8 @@ import { useWordAnchorFloating } from "@/hooks/useWordAnchorFloating";
 import { useCutRanges } from "@/hooks/useCutRanges";
 import { findActiveWordId, groupWordsBySpeaker } from "@/lib/transcript";
 import { isTypingTarget } from "@/lib/keyboard";
+import { useI18n } from "./I18nProvider";
+import { localizeRuntimeMessage } from "@/lib/i18n";
 
 const WordSpan = memo(function WordSpan({
   word,
@@ -56,6 +57,7 @@ const WordSpan = memo(function WordSpan({
   active: boolean;
   onClick: (word: Word, el: HTMLElement) => void;
 }) {
+  const { t } = useI18n();
   const placeholder = isDisfluencyPlaceholder(word.text);
   // The trailing space lives inside the span so that selection and deletion
   // highlights are continuous across words instead of breaking at each gap.
@@ -64,7 +66,7 @@ const WordSpan = memo(function WordSpan({
       data-wid={word.id}
       data-cut={cutOut ? "" : undefined}
       data-placeholder={placeholder ? "" : undefined}
-      title={placeholder ? "Detected hesitation (not transcribed) — cut with Remove filler words" : undefined}
+      title={placeholder ? t("transcript.hesitation") : undefined}
       onClick={(e) => onClick(word, e.currentTarget)}
       className={`py-0.5 cursor-pointer transition-colors duration-75 ${cutOut
         ? "word-deleted bg-red-50 text-red-600 line-through decoration-red-300 dark:bg-red-950/40 dark:text-red-400 dark:decoration-red-800"
@@ -91,11 +93,12 @@ const SplitMarker = memo(function SplitMarker({
   boundaryId: number;
   onJoin: (id: number) => void;
 }) {
+  const { t } = useI18n();
   return (
     <button
       type="button"
-      title="Clip split — click to join these clips"
-      aria-label="Join clips"
+      title={t("transcript.joinSplit")}
+      aria-label={t("transcript.joinClips")}
       onMouseDown={(e) => e.preventDefault()}
       onClick={() => onJoin(boundaryId)}
       className="group relative mx-0.5 inline-flex h-4 w-2 cursor-pointer select-none items-center justify-center align-middle"
@@ -103,13 +106,14 @@ const SplitMarker = memo(function SplitMarker({
       <span className="h-4 w-0.5 rounded-full bg-zinc-300 transition-colors group-hover:bg-zinc-600 dark:bg-zinc-600 dark:group-hover:bg-zinc-300" />
       <span className="pointer-events-none absolute -top-5 left-1/2 flex -translate-x-1/2 items-center gap-1 rounded-md bg-zinc-900 px-1.5 py-0.5 text-[10px] font-medium whitespace-nowrap text-white opacity-0 transition-opacity group-hover:opacity-100 dark:bg-zinc-100 dark:text-zinc-900">
         <Merge size={9} />
-        Join
+        {t("transcript.joinClips")}
       </span>
     </button>
   );
 });
 
 export default function TranscriptPanel() {
+  const { t } = useI18n();
   const words = useEditorStore((s) => s.words);
   const sceneBoundaries = useEditorStore((s) => s.sceneBoundaries);
   const duration = useEditorStore((s) => s.duration);
@@ -220,12 +224,12 @@ export default function TranscriptPanel() {
       const file = files?.[0];
       if (!file) return;
       if (!isTranscriptFile(file)) {
-        alert(TRANSCRIPT_FILE_ERROR);
+        alert(t("transcript.invalidFile"));
         return;
       }
       if (
         words.length > 0 &&
-        !confirm("Replace the current transcript with this file?")
+        !confirm(t("transcript.replaceConfirm"))
       ) {
         return;
       }
@@ -234,10 +238,14 @@ export default function TranscriptPanel() {
         importWords(imported.words, imported.speakers);
       } catch (err) {
         console.error(err);
-        alert(err instanceof Error ? err.message : "Could not read that transcript.");
+        alert(
+          err instanceof Error
+            ? localizeRuntimeMessage(err.message, t)
+            : t("error.readTranscript")
+        );
       }
     },
-    [words.length, importWords]
+    [words.length, importWords, t]
   );
 
   const cutSelection = useCallback(() => {
@@ -339,23 +347,23 @@ export default function TranscriptPanel() {
           rubber-band overscroll only carries the transcript, not the bar. */}
       <div className="absolute inset-x-0 top-0 z-10 flex h-10 items-center gap-2 border-b border-zinc-100/80 bg-white/75 px-3 backdrop-blur-md sm:px-4 dark:border-zinc-800/80 dark:bg-zinc-900/75">
         <span className="text-xs font-semibold uppercase tracking-wider text-zinc-400 dark:text-zinc-500">
-          Transcript
+          {t("transcript.header")}
         </span>
         <div className="ml-auto flex items-center gap-2">
           {deletedCount > 0 && (
             <span className="rounded-md bg-red-50 px-2 py-0.5 text-[9px] font-medium text-red-600 line-clamp-1 line-through dark:bg-red-950/40 dark:text-red-400">
-              {deletedCount} word{deletedCount === 1 ? "" : "s"}
+              {t("transcript.wordsDeleted", { count: deletedCount })}
             </span>
           )}
           {status === "ready" && <TranscriptToolsMenu />}
           {(status === "ready" || status === "error" || status === "transcribing") && (
             <>
               <label
-                title="Replace transcript from SRT, VTT, or JSON"
+                title={t("transcript.replace")}
                 className="flex cursor-pointer h-7 items-center gap-1.5 rounded-lg px-2 text-xs text-zinc-500 transition hover:bg-zinc-100 dark:text-zinc-400 dark:hover:bg-zinc-800"
               >
                 <ArrowUpFromLine size={14} />
-                <span className="hidden sm:inline">Import</span>
+                <span className="hidden sm:inline">{t("common.import")}</span>
                 <input
                   ref={importInputRef}
                   type="file"
@@ -373,7 +381,7 @@ export default function TranscriptPanel() {
           )}
           <button
             onClick={toggleShowDeleted}
-            title={showDeleted ? "Hide deleted words" : "Show deleted words"}
+            title={showDeleted ? t("transcript.hideDeleted") : t("transcript.showDeleted")}
             className="flex cursor-pointer h-7 items-center gap-1.5 rounded-lg px-2 text-xs text-zinc-500 transition hover:bg-zinc-100 dark:text-zinc-400 dark:hover:bg-zinc-800"
           >
             {showDeleted ? <Eye size={14} /> : <EyeOff size={14} />}
@@ -391,7 +399,7 @@ export default function TranscriptPanel() {
               <div className="w-full bg-zinc-50 p-2 dark:bg-zinc-800/60">
                 <div className="flex items-center gap-2">
                   <div className="h-4 w-4 animate-spin rounded-full border-2 border-neutral-500 border-t-transparent dark:border-neutral-400" />
-                  <p className="text-sm font-medium text-zinc-700 dark:text-zinc-200">{progress.message}</p>
+                  <p className="text-sm font-medium text-zinc-700 dark:text-zinc-200">{localizeRuntimeMessage(progress.message, t)}</p>
                   {progress.value !== null && (
                     <>
                       <div className="ml-auto w-[100px] h-1 overflow-hidden rounded-full bg-zinc-200 dark:bg-zinc-700">
@@ -418,13 +426,13 @@ export default function TranscriptPanel() {
 
           {status === "error" && (
             <div className="rounded-xl border border-red-200 bg-red-50 p-4 text-sm text-red-600 dark:border-red-900/30 dark:bg-red-950/30 dark:text-red-900">
-              {error}
+              {localizeRuntimeMessage(error, t)}
             </div>
           )}
 
           {status === "ready" && words.length === 0 && (
               <p className="mt-2 flex items-center gap-1 text-sm font-medium text-zinc-500 dark:text-zinc-500">
-                <VolumeOff size={16} /> No speech detected, make sure this file has audio or import a transcript.
+                <VolumeOff size={16} /> {t("transcript.noSpeech")}
               </p>
           )}
 
@@ -484,7 +492,7 @@ export default function TranscriptPanel() {
                     className="flex cursor-pointer items-center gap-1.5 rounded-lg px-2.5 py-1.5 text-[13px] font-medium text-zinc-700 transition hover:bg-red-50 hover:text-red-600 dark:text-zinc-200 dark:hover:bg-red-950/50 dark:hover:text-red-400"
                   >
                     <Scissors size={13} />
-                    Cut
+                    {t("transcript.cut")}
                   </button>
                 )}
                 {selection.anyDeleted && (
@@ -493,7 +501,7 @@ export default function TranscriptPanel() {
                     className="flex cursor-pointer items-center gap-1.5 rounded-lg px-2.5 py-1.5 text-[13px] font-medium text-zinc-700 transition hover:bg-emerald-50 hover:text-emerald-600 dark:text-zinc-200 dark:hover:bg-emerald-950/40 dark:hover:text-emerald-400"
                   >
                     <RotateCcw size={13} />
-                    Restore
+                    {t("common.restore")}
                   </button>
                 )}
                 <button
@@ -501,7 +509,7 @@ export default function TranscriptPanel() {
                   className="flex cursor-pointer items-center gap-1.5 rounded-lg px-2.5 py-1.5 text-[13px] font-medium text-zinc-700 transition hover:bg-zinc-100 dark:text-zinc-200 dark:hover:bg-zinc-700"
                 >
                   <Pencil size={13} />
-                  Correct
+                  {t("transcript.correct")}
                 </button>
                 <SelectionSpeakerButton onClick={openSpeakerAssign} />
               </div>
@@ -527,7 +535,7 @@ export default function TranscriptPanel() {
                 style={correctStyles}
               >
                 <div className="mb-2 flex items-center justify-between">
-                  <span className="text-[13px] font-semibold text-zinc-800 dark:text-zinc-100">Correct</span>
+                  <span className="text-[13px] font-semibold text-zinc-800 dark:text-zinc-100">{t("transcript.correct")}</span>
                   <button
                     onClick={closeCorrect}
                     className="flex h-6 w-6 items-center justify-center rounded-md text-zinc-400 transition hover:bg-zinc-100 hover:text-zinc-700 dark:hover:bg-zinc-700 dark:hover:text-zinc-200"
@@ -552,7 +560,7 @@ export default function TranscriptPanel() {
                     disabled={correctText.trim().length === 0}
                     className="cursor-pointer flex h-8 items-center rounded-full bg-zinc-900 px-4 text-[13px] font-medium text-white transition hover:bg-zinc-700 disabled:opacity-40 dark:bg-zinc-100 dark:text-zinc-900 dark:hover:bg-zinc-200"
                   >
-                    Correct
+                    {t("transcript.correct")}
                   </button>
                 </div>
               </div>
@@ -566,13 +574,13 @@ export default function TranscriptPanel() {
         <button
           type="button"
           onClick={resumeFollowPlayhead}
-          title="Scroll with the playhead"
+          title={t("transcript.scrollWithPlayhead")}
           className="absolute bottom-5 left-1/2 z-20 flex -translate-x-1/2 cursor-pointer items-center gap-1.5 rounded-full border border-zinc-200 bg-white/95 px-3 py-1.5 text-xs font-medium text-zinc-700 backdrop-blur-sm transition hover:bg-zinc-50 dark:border-zinc-700 dark:bg-zinc-800/95 dark:text-zinc-200 dark:hover:bg-zinc-700"
         >
           {followDirection === "up" && <ArrowUp size={13} />}
           {followDirection === "down" && <ArrowDown size={13} />}
           {followDirection === null && <ChevronLast size={13} />}
-          Follow playhead
+          {t("transcript.follow")}
         </button>
       )}
       <TranscriptScrollIndicator
diff --git a/components/TranscriptToolsMenu.tsx b/components/TranscriptToolsMenu.tsx
index 8f9fafe..4aea720 100644
--- a/components/TranscriptToolsMenu.tsx
+++ b/components/TranscriptToolsMenu.tsx
@@ -10,6 +10,8 @@ import {
   MIN_SILENCE_DURATION,
 } from "@/lib/silences";
 import Popover, { PopoverContent, PopoverTrigger } from "./Popover";
+import { useI18n } from "./I18nProvider";
+import type { MessageKey } from "@/lib/i18n";
 
 /**
  * What every tool gets to work with: the targets it would act on, plus the
@@ -29,8 +31,8 @@ type ToolContext = {
 
 type ToolDef = {
   key: string;
-  label: string;
-  title: string;
+  labelKey: MessageKey;
+  titleKey: MessageKey;
   Icon: LucideIcon;
   /** How much the tool would touch. Zero hides it from the menu entirely. */
   count: (ctx: ToolContext) => number;
@@ -41,32 +43,32 @@ type ToolDef = {
 const TOOLS: ToolDef[] = [
   {
     key: "remove-fillers",
-    label: "Remove filler words",
-    title: 'Cut filler words ("um", "uh", "...", …) from the video',
+    labelKey: "tools.removeFillers",
+    titleKey: "tools.removeFillersTitle",
     Icon: WandSparkles,
     count: (ctx) => ctx.fillerIds.length,
     run: (ctx) => ctx.deleteWords(ctx.fillerIds),
   },
   {
     key: "restore-fillers",
-    label: "Restore filler words",
-    title: "Bring every cut filler word back",
+    labelKey: "tools.restoreFillers",
+    titleKey: "tools.restoreFillersTitle",
     Icon: Undo2,
     count: (ctx) => ctx.deletedFillerIds.length,
     run: (ctx) => ctx.restoreWords(ctx.deletedFillerIds),
   },
   {
     key: "remove-silences",
-    label: "Remove silences",
-    title: `Cut pauses and silences (≥${MIN_SILENCE_DURATION}s) from the video`,
+    labelKey: "tools.removeSilences",
+    titleKey: "tools.removeSilencesTitle",
     Icon: VolumeX,
     count: (ctx) => ctx.silenceRanges.length,
     run: (ctx) => ctx.cutRanges(ctx.silenceRanges),
   },
   {
     key: "restore-silences",
-    label: "Restore silences",
-    title: "Bring every cut pause back",
+    labelKey: "tools.restoreSilences",
+    titleKey: "tools.restoreSilencesTitle",
     Icon: Undo2,
     count: (ctx) => ctx.silenceCuts.length,
     run: (ctx) => ctx.restoreRanges(ctx.silenceCuts),
@@ -79,6 +81,7 @@ const TOOLS: ToolDef[] = [
  * right now" — and the menu hides itself when that leaves none.
  */
 export default function TranscriptToolsMenu() {
+  const { t } = useI18n();
   const words = useEditorStore((s) => s.words);
   const duration = useEditorStore((s) => s.duration);
   const manualCuts = useEditorStore((s) => s.manualCuts);
@@ -131,12 +134,12 @@ export default function TranscriptToolsMenu() {
             aria-haspopup="menu"
             aria-expanded={open}
             aria-controls={panelId}
-            title="Bulk transcript cleanups"
+            title={t("tools.bulk")}
             onClick={() => setOpen((v) => !v)}
             className="flex h-7 cursor-pointer items-center gap-1.5 rounded-lg px-2 text-xs text-zinc-500 transition hover:bg-zinc-100 dark:text-zinc-400 dark:hover:bg-zinc-800"
           >
             <Zap size={14} />
-            <span className="hidden sm:inline">Tools</span>
+            <span className="hidden sm:inline">{t("common.tools")}</span>
             <span className="rounded-full bg-zinc-100 px-1.5 py-0.5 text-[9px] font-medium tabular-nums text-zinc-600 dark:bg-zinc-700 dark:text-zinc-300">
               {available.length}
             </span>
@@ -146,7 +149,7 @@ export default function TranscriptToolsMenu() {
         <PopoverContent
           id={panelId}
           role="menu"
-          aria-label="Tools"
+          aria-label={t("common.tools")}
           className="z-40 w-[15rem] overflow-hidden p-1.5"
         >
           {available.map(({ tool, count }) => (
@@ -154,7 +157,9 @@ export default function TranscriptToolsMenu() {
               key={tool.key}
               type="button"
               role="menuitem"
-              title={tool.title}
+              title={t(tool.titleKey, {
+                seconds: MIN_SILENCE_DURATION,
+              })}
               onClick={() => {
                 tool.run(ctx);
                 setOpen(false);
@@ -164,7 +169,7 @@ export default function TranscriptToolsMenu() {
               <span className="shrink-0 text-zinc-400 dark:text-zinc-500">
                 <tool.Icon size={14} />
               </span>
-              <span className="flex-1">{tool.label}</span>
+              <span className="flex-1">{t(tool.labelKey)}</span>
               <span className="shrink-0 text-[11px] tabular-nums text-zinc-400 dark:text-zinc-500">
                 {count}
               </span>
diff --git a/components/UploadScreen.tsx b/components/UploadScreen.tsx
index 9077b51..1122a90 100644
--- a/components/UploadScreen.tsx
+++ b/components/UploadScreen.tsx
@@ -27,13 +27,17 @@ import { useCrossOriginIsolated } from "@/hooks/useCrossOriginIsolated";
 import { detectMediaKind, MEDIA_ACCEPT } from "@/lib/media";
 import { formatTime } from "@/lib/edits";
 import {
-  formatRelativeTime,
   listProjects,
   type ProjectMeta,
 } from "@/lib/projects";
 import { isElectron } from "@/lib/platform";
 import { useEditorStore } from "@/lib/store";
 import type { SpeakerInfo, Word } from "@/lib/types";
+import { useI18n } from "./I18nProvider";
+import {
+  formatRelativeTime,
+  localizeRuntimeMessage,
+} from "@/lib/i18n";
 
 // The three media cards that stand in for the upload icon. Each carries its
 // resting transform plus the fanned-out one, applied either on hover (via the
@@ -97,11 +101,12 @@ function RecentProjects({
   onOpen: (id: string) => void;
   onRemove: (id: string) => void;
 }) {
+  const { locale, t } = useI18n();
   if (projects.length === 0) return null;
   return (
     <div className="mt-6">
       <p className="mb-2 text-[11px] font-medium tracking-wide text-zinc-400 dark:text-zinc-500">
-        Recent
+        {t("upload.recentProjects")}
       </p>
       <ul className="divide-y divide-zinc-100 overflow-hidden rounded-xl border border-zinc-200 bg-white/80 dark:divide-zinc-800 dark:border-zinc-700 dark:bg-zinc-900/80">
         {projects.map((p) => {
@@ -122,9 +127,9 @@ function RecentProjects({
                       {p.name}
                     </span>
                     <span className="mt-0.5 block text-[11px] text-zinc-400 dark:text-zinc-500">
-                      {formatRelativeTime(p.updatedAt)}
+                      {formatRelativeTime(locale, p.updatedAt)}
                       {p.duration > 0 ? ` · ${formatTime(p.duration)}` : ""}
-                      {` · ${p.mediaKind}`}
+                      {` · ${p.mediaKind === "audio" ? t("export.audio") : t("export.video")}`}
                     </span>
                   </span>
                   {opening && (
@@ -133,7 +138,7 @@ function RecentProjects({
                 </button>
                 <button
                   type="button"
-                  title="Remove from recent"
+                  title={t("upload.removeRecent")}
                   disabled={busyId !== null}
                   onClick={(e) => {
                     e.stopPropagation();
@@ -173,6 +178,7 @@ export default function UploadScreen({
   const pendingTranscript = useEditorStore((s) => s.pendingTranscript);
   const openProject = useEditorStore((s) => s.openProject);
   const removeProject = useEditorStore((s) => s.removeProject);
+  const { t } = useI18n();
 
   const refreshProjects = useCallback(async () => {
     try {
@@ -205,13 +211,13 @@ export default function UploadScreen({
       const file = files?.[0];
       if (!file) return;
       if (!detectMediaKind(file)) {
-        alert("Please choose a video or audio file.");
+        alert(t("editor.chooseMedia"));
         return;
       }
       const { source, pendingTranscript: pending } = useEditorStore.getState();
       if (source === "import") {
         if (!pending) {
-          alert("Choose a transcript file from the source menu first.");
+          alert(t("editor.chooseTranscript"));
           return;
         }
         onFile(file, { words: pending.words, speakers: pending.speakers });
@@ -219,7 +225,7 @@ export default function UploadScreen({
       }
       onFile(file);
     },
-    [onFile, ready]
+    [onFile, ready, t]
   );
 
   const handleOpen = useCallback(
@@ -230,13 +236,17 @@ export default function UploadScreen({
         await openProject(id);
       } catch (err) {
         console.error(err);
-        alert(err instanceof Error ? err.message : "Could not open that project.");
+        alert(
+          err instanceof Error
+            ? localizeRuntimeMessage(err.message, t)
+            : t("error.openProject")
+        );
         await refreshProjects();
       } finally {
         setBusyId(null);
       }
     },
-    [openProject, ready, refreshProjects]
+    [openProject, ready, refreshProjects, t]
   );
 
   const handleRemove = useCallback(
@@ -246,10 +256,10 @@ export default function UploadScreen({
         await refreshProjects();
       } catch (err) {
         console.error(err);
-        alert("Could not remove that project.");
+        alert(t("error.removeProject"));
       }
     },
-    [removeProject, refreshProjects]
+    [removeProject, refreshProjects, t]
   );
 
   return (
@@ -278,7 +288,7 @@ export default function UploadScreen({
               <div className="flex flex-wrap items-center gap-2 sm:justify-end">
                 <SettingsMenu />
                 <div className="h-5 w-px bg-zinc-200 dark:bg-zinc-700 mr-1" />
-                <ModelSelector groupLabel="Transcript source">
+                <ModelSelector groupLabel={t("model.transcriptSource")}>
                   {MODEL_ORDER.map((id) => (
                     <ModelOption key={id} id={id} />
                   ))}
@@ -345,32 +355,34 @@ export default function UploadScreen({
             {isolation === "unavailable" ? (
               <>
                 <p className="text-[15px] font-medium text-zinc-800 dark:text-zinc-100">
-                  This browser can&apos;t run the editor
+                  {t("upload.unsupported")}
                 </p>
                 <p className="mt-1 max-w-sm text-[13px] leading-relaxed text-zinc-400 dark:text-zinc-500">
-                  Editing needs SharedArrayBuffer, which requires a cross-origin-isolated page.
-                  Try a recent Chrome, Edge, Safari or Firefox over HTTPS.
+                  {t("upload.unsupportedHelp")}
                 </p>
               </>
             ) : ready ? (
               <>
                 <p className="text-[15px] font-medium text-zinc-800 dark:text-zinc-100">
-                  Drop a video or audio file here, or{" "}
-                  <span className="text-neutral-600 dark:text-neutral-300">browse</span>
+                  {t("upload.dropOrBrowse", {
+                    browse: t("upload.browse"),
+                  })}
                 </p>
                 <p className="mt-1 text-[13px] text-zinc-400 dark:text-zinc-500">
                   {source === "import"
                     ? pendingTranscript
-                      ? `Will use ${pendingTranscript.name} · MP4, WebM, MOV, MP3, WAV, …`
-                      : "Pick a transcript in the menu above, then drop your media"
-                    : "MP4, WebM, MOV, MP3, WAV, M4A, …"}
+                      ? t("upload.willUseTranscript", {
+                          name: pendingTranscript.name,
+                        })
+                      : t("upload.chooseTranscriptFirst")
+                    : t("upload.mediaFormats")}
                 </p>
               </>
             ) : (
               <>
-                <p className="text-[15px] font-medium text-zinc-800 dark:text-zinc-100">Getting things ready</p>
+                <p className="text-[15px] font-medium text-zinc-800 dark:text-zinc-100">{t("upload.gettingReady")}</p>
                 <p className="mt-1 text-[13px] text-zinc-400 dark:text-zinc-500">
-                  Setting up the media engine, this only happens once.
+                  {t("upload.gettingReadyHelp")}
                 </p>
               </>
             )}
@@ -399,9 +411,9 @@ export default function UploadScreen({
 
           {!isElectron && <div className="mt-6 grid grid-cols-1 gap-3 sm:grid-cols-3">
             {[
-              { icon: Type, title: "Transcribe", text: "Whisper locally, or import SRT / VTT." },
-              { icon: Scissors, title: "Edit", text: "Select words and hit delete to edit." },
-              { icon: Clapperboard, title: "Export", text: "Render the final cut to MP4 or M4A." },
+              { icon: Type, title: t("upload.transcribeTitle"), text: t("upload.transcribeText") },
+              { icon: Scissors, title: t("upload.editTitle"), text: t("upload.editText") },
+              { icon: Clapperboard, title: t("upload.exportTitle"), text: t("upload.exportText") },
             ].map(({ icon: Icon, title, text }) => (
               <div key={title} className="rounded-xl border border-zinc-200 bg-white/70 p-4 dark:border-zinc-700 dark:bg-zinc-900/70">
                 <Icon size={16} className="mb-2 text-neutral-500 dark:text-neutral-400" />
diff --git a/electron/locale.ts b/electron/locale.ts
new file mode 100644
index 0000000..10b3122
--- /dev/null
+++ b/electron/locale.ts
@@ -0,0 +1,108 @@
+export type DesktopLocale = "en" | "zh-CN";
+
+let currentLocale: DesktopLocale = "en";
+
+const en = {
+  file: "File",
+  openProject: "Open Project…",
+  reopenLast: "Reopen Last Project",
+  recentProjects: "Recent Projects",
+  noRecent: "No Recent Projects",
+  clearRecent: "Clear Recent Projects",
+  edit: "Edit",
+  undo: "Undo",
+  redo: "Redo",
+  cut: "Cut",
+  copy: "Copy",
+  paste: "Paste",
+  pasteMatch: "Paste and Match Style",
+  delete: "Delete",
+  selectAll: "Select All",
+  view: "View",
+  reload: "Reload",
+  forceReload: "Force Reload",
+  devTools: "Toggle Developer Tools",
+  resetZoom: "Actual Size",
+  zoomIn: "Zoom In",
+  zoomOut: "Zoom Out",
+  fullscreen: "Toggle Full Screen",
+  window: "Window",
+  minimize: "Minimize",
+  zoom: "Zoom",
+  front: "Bring All to Front",
+  close: "Close Window",
+  quit: "Quit Rescript",
+  about: "About Rescript",
+  services: "Services",
+  hide: "Hide Rescript",
+  hideOthers: "Hide Others",
+  unhide: "Show All",
+  restart: "Restart",
+  later: "Later",
+  updateTitle: "Update available",
+  updateMessage: "Rescript {version} is ready to install.",
+  updateDetail: "Restart now to apply the update.",
+} as const;
+
+type DesktopMessageKey = keyof typeof en;
+
+const zhCN: Record<DesktopMessageKey, string> = {
+  file: "文件",
+  openProject: "打开项目…",
+  reopenLast: "重新打开上一个项目",
+  recentProjects: "最近项目",
+  noRecent: "没有最近项目",
+  clearRecent: "清除最近项目",
+  edit: "编辑",
+  undo: "撤销",
+  redo: "重做",
+  cut: "剪切",
+  copy: "复制",
+  paste: "粘贴",
+  pasteMatch: "粘贴并匹配样式",
+  delete: "删除",
+  selectAll: "全选",
+  view: "视图",
+  reload: "重新加载",
+  forceReload: "强制重新加载",
+  devTools: "切换开发者工具",
+  resetZoom: "实际大小",
+  zoomIn: "放大",
+  zoomOut: "缩小",
+  fullscreen: "切换全屏",
+  window: "窗口",
+  minimize: "最小化",
+  zoom: "缩放",
+  front: "前置全部窗口",
+  close: "关闭窗口",
+  quit: "退出 Rescript",
+  about: "关于 Rescript",
+  services: "服务",
+  hide: "隐藏 Rescript",
+  hideOthers: "隐藏其他应用",
+  unhide: "全部显示",
+  restart: "立即重启",
+  later: "稍后",
+  updateTitle: "有可用更新",
+  updateMessage: "Rescript {version} 已准备好安装。",
+  updateDetail: "立即重启以应用更新。",
+};
+
+export function resolveDesktopLocale(value: string): DesktopLocale {
+  const locale = value.toLowerCase();
+  return locale === "zh" || locale.startsWith("zh-") ? "zh-CN" : "en";
+}
+
+export function setDesktopLocale(locale: DesktopLocale): void {
+  currentLocale = locale;
+}
+
+export function desktopText(
+  key: DesktopMessageKey,
+  params: Record<string, string | number> = {}
+): string {
+  const template = (currentLocale === "zh-CN" ? zhCN : en)[key];
+  return template.replace(/\{(\w+)\}/g, (token, name: string) =>
+    Object.prototype.hasOwnProperty.call(params, name) ? String(params[name]) : token
+  );
+}
diff --git a/electron/main.ts b/electron/main.ts
index 3440e55..e503374 100644
--- a/electron/main.ts
+++ b/electron/main.ts
@@ -10,7 +10,7 @@ import {
 } from "electron";
 import { join, normalize, extname } from "node:path";
 import { pathToFileURL } from "node:url";
-import { existsSync, statSync } from "node:fs";
+import { existsSync, readFileSync, statSync, writeFileSync } from "node:fs";
 import { initMainSentry, setMainTelemetryEnabled } from "./sentry";
 import { initAutoUpdater } from "./updater";
 import {
@@ -19,6 +19,11 @@ import {
   type MenuCommand,
   type RecentProject,
 } from "./menu";
+import {
+  resolveDesktopLocale,
+  setDesktopLocale,
+  type DesktopLocale,
+} from "./locale";
 
 const isDev = !app.isPackaged;
 const DEV_SERVER_URL = process.env.ELECTRON_START_URL ?? "http://localhost:3000";
@@ -26,13 +31,19 @@ const isMac = process.platform === "darwin";
 
 type WindowMode = "compact" | "expanded";
 
-/** The shell has two resting sizes: a small window for the upload screen, and a
- *  roomy one once the editor (transcript + preview + timeline) takes over. */
-const WINDOW_SIZES: Record<WindowMode, { width: number; height: number }> = {
-  compact: { width: 560, height: 400 },
-  expanded: { width: 1080, height: 740 },
+/** Start roomy enough for the three-pane editor; subsequent launches restore
+ *  the user's own normal bounds instead of forcing upload/editor sizes. */
+const DEFAULT_WINDOW_SIZE = { width: 1280, height: 820 };
+const MIN_SIZE = { width: 720, height: 480 };
+const WINDOW_STATE_FILE = "window-state.json";
+
+type StoredWindowState = {
+  x: number;
+  y: number;
+  width: number;
+  height: number;
+  maximized: boolean;
 };
-const MIN_SIZE = { width: 560, height: 400 };
 
 /** Height of the in-page drag strip (`h-12`), used to centre the traffic lights. */
 const TITLE_BAR_HEIGHT = 48;
@@ -179,39 +190,72 @@ function clamp(value: number, min: number, max: number): number {
   return Math.min(Math.max(value, min), max);
 }
 
-/** Resize a window to the given mode's resting size, keeping it centred on
- *  wherever the user left it rather than snapping to a corner. */
-function applyWindowMode(win: BrowserWindow, mode: WindowMode): void {
-  if (windowModes.get(win) === mode) return;
-  windowModes.set(win, mode);
-  // A maximized or full-screen window is already the size the user asked for.
-  if (win.isFullScreen() || win.isMaximized()) return;
-
-  const current = win.getBounds();
-  const { workArea } = screen.getDisplayMatching(current);
-  const width = Math.min(WINDOW_SIZES[mode].width, workArea.width);
-  const height = Math.min(WINDOW_SIZES[mode].height, workArea.height);
-  win.setBounds(
-    {
+function isStoredWindowState(value: unknown): value is StoredWindowState {
+  if (!value || typeof value !== "object") return false;
+  const state = value as Partial<StoredWindowState>;
+  return (
+    [state.x, state.y, state.width, state.height].every(Number.isFinite) &&
+    typeof state.maximized === "boolean"
+  );
+}
+
+/** Restore saved bounds onto the closest current display. This also recovers
+ *  from a monitor being disconnected between launches. */
+function loadWindowState(): StoredWindowState {
+  const primaryWorkArea = screen.getPrimaryDisplay().workArea;
+  const fallbackWidth = Math.min(DEFAULT_WINDOW_SIZE.width, primaryWorkArea.width);
+  const fallbackHeight = Math.min(DEFAULT_WINDOW_SIZE.height, primaryWorkArea.height);
+  const fallback: StoredWindowState = {
+    width: fallbackWidth,
+    height: fallbackHeight,
+    x: Math.round(primaryWorkArea.x + (primaryWorkArea.width - fallbackWidth) / 2),
+    y: Math.round(primaryWorkArea.y + (primaryWorkArea.height - fallbackHeight) / 2),
+    maximized: false,
+  };
+
+  const statePath = join(app.getPath("userData"), WINDOW_STATE_FILE);
+  if (!existsSync(statePath)) return fallback;
+  try {
+    const parsed: unknown = JSON.parse(readFileSync(statePath, "utf8"));
+    if (!isStoredWindowState(parsed)) return fallback;
+    const workArea = screen.getDisplayMatching(parsed).workArea;
+    const width = Math.min(Math.max(Math.round(parsed.width), MIN_SIZE.width), workArea.width);
+    const height = Math.min(
+      Math.max(Math.round(parsed.height), MIN_SIZE.height),
+      workArea.height
+    );
+    return {
       width,
       height,
-      x: Math.round(
-        clamp(
-          current.x + (current.width - width) / 2,
-          workArea.x,
-          workArea.x + workArea.width - width
-        )
-      ),
-      y: Math.round(
-        clamp(
-          current.y + (current.height - height) / 2,
-          workArea.y,
-          workArea.y + workArea.height - height
-        )
-      ),
-    },
-    true // animate (macOS)
-  );
+      x: Math.round(clamp(parsed.x, workArea.x, workArea.x + workArea.width - width)),
+      y: Math.round(clamp(parsed.y, workArea.y, workArea.y + workArea.height - height)),
+      maximized: parsed.maximized,
+    };
+  } catch (error) {
+    console.warn("Could not restore the saved window bounds.", error);
+    return fallback;
+  }
+}
+
+function saveWindowState(win: BrowserWindow): void {
+  if (win.isDestroyed()) return;
+  const bounds = win.getNormalBounds();
+  const state: StoredWindowState = {
+    x: bounds.x,
+    y: bounds.y,
+    width: bounds.width,
+    height: bounds.height,
+    maximized: win.isMaximized(),
+  };
+  try {
+    writeFileSync(
+      join(app.getPath("userData"), WINDOW_STATE_FILE),
+      JSON.stringify(state),
+      "utf8"
+    );
+  } catch (error) {
+    console.warn("Could not save the window bounds.", error);
+  }
 }
 
 /** Set once the app is really terminating, so the close interception below
@@ -219,8 +263,12 @@ function applyWindowMode(win: BrowserWindow, mode: WindowMode): void {
 let quitting = false;
 
 function createWindow(): BrowserWindow {
+  const restoredState = loadWindowState();
   const win = new BrowserWindow({
-    ...WINDOW_SIZES.compact,
+    x: restoredState.x,
+    y: restoredState.y,
+    width: restoredState.width,
+    height: restoredState.height,
     minWidth: MIN_SIZE.width,
     minHeight: MIN_SIZE.height,
     // Light by default — appearance is a user preference in the renderer.
@@ -248,7 +296,26 @@ function createWindow(): BrowserWindow {
   });
   windowModes.set(win, "compact");
 
-  win.once("ready-to-show", () => win.show());
+  win.once("ready-to-show", () => {
+    if (restoredState.maximized) win.maximize();
+    win.show();
+  });
+
+  let saveBoundsTimer: ReturnType<typeof setTimeout> | null = null;
+  const scheduleWindowStateSave = () => {
+    if (saveBoundsTimer) clearTimeout(saveBoundsTimer);
+    saveBoundsTimer = setTimeout(() => {
+      saveBoundsTimer = null;
+      saveWindowState(win);
+    }, 250);
+  };
+  win.on("resize", scheduleWindowStateSave);
+  win.on("move", scheduleWindowStateSave);
+  win.on("maximize", scheduleWindowStateSave);
+  win.on("unmaximize", scheduleWindowStateSave);
+  win.on("closed", () => {
+    if (saveBoundsTimer) clearTimeout(saveBoundsTimer);
+  });
 
   // A reload tears down the listener the renderer registered; make it re-announce.
   win.webContents.on("did-start-navigation", (event) => {
@@ -262,6 +329,7 @@ function createWindow(): BrowserWindow {
   // next close (already on the upload screen) is a real close. Guarded on the
   // renderer being live, so an unresponsive page can still be closed.
   win.on("close", (event) => {
+    saveWindowState(win);
     if (quitting) return;
     if (windowModes.get(win) !== "expanded") return;
     if (!readyRenderers.has(win.webContents)) return;
@@ -323,7 +391,7 @@ if (!gotLock) {
   ipcMain.on("window:set-mode", (event, mode: unknown) => {
     if (mode !== "compact" && mode !== "expanded") return;
     const win = BrowserWindow.fromWebContents(event.sender);
-    if (win) applyWindowMode(win, mode);
+    if (win) windowModes.set(win, mode);
   });
   ipcMain.handle(
     "window:is-full-screen",
@@ -334,6 +402,11 @@ if (!gotLock) {
   ipcMain.on("telemetry:set-enabled", (_event, value: unknown) => {
     setMainTelemetryEnabled(value === true);
   });
+  ipcMain.on("ui:set-locale", (_event, value: unknown) => {
+    if (value !== "en" && value !== "zh-CN") return;
+    setDesktopLocale(value as DesktopLocale);
+    buildAppMenu();
+  });
   // The saved projects live in the renderer's IndexedDB; it pushes a snapshot
   // whenever the list changes so the File menu can list them.
   ipcMain.on("menu:set-recents", (_event, value: unknown) => {
@@ -359,6 +432,7 @@ if (!gotLock) {
   });
 
   app.whenReady().then(() => {
+    setDesktopLocale(resolveDesktopLocale(app.getLocale()));
     if (!isDev) registerAppProtocol();
     buildAppMenu(dispatchMenuCommand);
     createWindow();
diff --git a/electron/menu.ts b/electron/menu.ts
index 283011b..fcbffac 100644
--- a/electron/menu.ts
+++ b/electron/menu.ts
@@ -1,4 +1,5 @@
 import { app, Menu, type MenuItemConstructorOptions } from "electron";
+import { desktopText as t } from "./locale";
 
 const isMac = process.platform === "darwin";
 
@@ -34,15 +35,15 @@ function send(command: MenuCommand): void {
 function fileMenu(): MenuItemConstructorOptions {
   const [last] = recents;
   return {
-    label: "File",
+    label: t("file"),
     submenu: [
       {
-        label: "Open Project…",
+        label: t("openProject"),
         accelerator: "CmdOrCtrl+O",
         click: () => send({ type: "open-file" }),
       },
       {
-        label: "Reopen Last Project",
+        label: t("reopenLast"),
         accelerator: "Shift+CmdOrCtrl+O",
         enabled: last !== undefined,
         click: () => {
@@ -50,10 +51,10 @@ function fileMenu(): MenuItemConstructorOptions {
         },
       },
       {
-        label: "Recent Projects",
+        label: t("recentProjects"),
         submenu:
           recents.length === 0
-            ? [{ label: "No Recent Projects", enabled: false }]
+            ? [{ label: t("noRecent"), enabled: false }]
             : [
                 ...recents.slice(0, MAX_RECENT_ITEMS).map((p) => ({
                   label: p.name,
@@ -61,13 +62,15 @@ function fileMenu(): MenuItemConstructorOptions {
                 })),
                 { type: "separator" as const },
                 {
-                  label: "Clear Recent Projects",
+                  label: t("clearRecent"),
                   click: () => send({ type: "clear-recents" }),
                 },
               ],
       },
       { type: "separator" },
-      isMac ? { role: "close" } : { role: "quit" },
+      isMac
+        ? { role: "close", label: t("close") }
+        : { role: "quit", label: t("quit") },
     ],
   };
 }
@@ -81,69 +84,69 @@ function template(): MenuItemConstructorOptions[] {
           {
             label: app.name,
             submenu: [
-              { role: "about" },
+              { role: "about", label: t("about") },
               { type: "separator" },
-              { role: "services" },
+              { role: "services", label: t("services") },
               { type: "separator" },
-              { role: "hide" },
-              { role: "hideOthers" },
-              { role: "unhide" },
+              { role: "hide", label: t("hide") },
+              { role: "hideOthers", label: t("hideOthers") },
+              { role: "unhide", label: t("unhide") },
               { type: "separator" },
-              { role: "quit" },
+              { role: "quit", label: t("quit") },
             ],
           },
         ] satisfies MenuItemConstructorOptions[])
       : []),
     fileMenu(),
     {
-      label: "Edit",
+      label: t("edit"),
       submenu: [
-        { role: "undo" },
-        { role: "redo" },
+        { role: "undo", label: t("undo") },
+        { role: "redo", label: t("redo") },
         { type: "separator" },
-        { role: "cut" },
-        { role: "copy" },
-        { role: "paste" },
+        { role: "cut", label: t("cut") },
+        { role: "copy", label: t("copy") },
+        { role: "paste", label: t("paste") },
         ...(isMac
           ? ([
-              { role: "pasteAndMatchStyle" },
-              { role: "delete" },
-              { role: "selectAll" },
+              { role: "pasteAndMatchStyle", label: t("pasteMatch") },
+              { role: "delete", label: t("delete") },
+              { role: "selectAll", label: t("selectAll") },
             ] satisfies MenuItemConstructorOptions[])
           : ([
-              { role: "delete" },
+              { role: "delete", label: t("delete") },
               { type: "separator" },
-              { role: "selectAll" },
+              { role: "selectAll", label: t("selectAll") },
             ] satisfies MenuItemConstructorOptions[])),
       ],
     },
     {
-      label: "View",
+      label: t("view"),
       submenu: [
-        { role: "reload" },
-        { role: "forceReload" },
-        { role: "toggleDevTools" },
+        { role: "reload", label: t("reload") },
+        { role: "forceReload", label: t("forceReload") },
+        { role: "toggleDevTools", label: t("devTools") },
         { type: "separator" },
-        { role: "resetZoom" },
-        { role: "zoomIn" },
-        { role: "zoomOut" },
+        { role: "resetZoom", label: t("resetZoom") },
+        { role: "zoomIn", label: t("zoomIn") },
+        { role: "zoomOut", label: t("zoomOut") },
         { type: "separator" },
-        { role: "togglefullscreen" },
+        { role: "togglefullscreen", label: t("fullscreen") },
       ],
     },
     {
-      label: "Window",
+      label: t("window"),
       submenu: [
-        { role: "minimize" },
-        { role: "zoom" },
+        { role: "minimize", label: t("minimize") },
+        { role: "zoom", label: t("zoom") },
         ...(isMac
           ? ([
               { type: "separator" },
-              { role: "front" },
+              { role: "front", label: t("front") },
               { type: "separator" },
-              { role: "window" },
+              { role: "window", label: t("window") },
             ] satisfies MenuItemConstructorOptions[])
-          : ([{ role: "close" }] satisfies MenuItemConstructorOptions[])),
+          : ([{ role: "close", label: t("close") }] satisfies MenuItemConstructorOptions[])),
       ],
     },
   ];
diff --git a/electron/preload.ts b/electron/preload.ts
index 2b54b7e..f02728a 100644
--- a/electron/preload.ts
+++ b/electron/preload.ts
@@ -4,7 +4,7 @@ import { contextBridge, ipcRenderer, type IpcRendererEvent } from "electron";
  * Minimal bridge for the renderer. Rescript's UI is still a normal web
  * surface; we only expose host metadata so the page can adapt chrome / skip
  * the COI service worker (headers come from the app:// protocol instead),
- * plus the few window controls the page drives (sizing, title-bar state).
+ * plus the few window controls the page drives (project mode, title-bar state).
  */
 contextBridge.exposeInMainWorld("rescriptDesktop", {
   platform: process.platform as NodeJS.Platform,
@@ -13,7 +13,7 @@ contextBridge.exposeInMainWorld("rescriptDesktop", {
     chrome: process.versions.chrome,
     node: process.versions.node,
   },
-  /** Switch between the compact upload window and the full editor window. */
+  /** Tell the host whether a project is open so native Close keeps its two-step behavior. */
   setWindowMode: (mode: "compact" | "expanded") => {
     ipcRenderer.send("window:set-mode", mode);
   },
@@ -24,6 +24,10 @@ contextBridge.exposeInMainWorld("rescriptDesktop", {
   setTelemetryEnabled: (enabled: boolean) => {
     ipcRenderer.send("telemetry:set-enabled", enabled);
   },
+  /** Keep native menus and dialogs in sync with the renderer preference. */
+  setUiLocale: (locale: "en" | "zh-CN") => {
+    ipcRenderer.send("ui:set-locale", locale);
+  },
   /**
    * Publish the saved-project list (newest first) so the main process can draw
    * it under File › Recent Projects. Only id + name are sent.
diff --git a/electron/updater.ts b/electron/updater.ts
index 68fc483..29210a6 100644
--- a/electron/updater.ts
+++ b/electron/updater.ts
@@ -1,6 +1,7 @@
 import { app, BrowserWindow, dialog } from "electron";
 import log from "electron-log";
 import { autoUpdater } from "electron-updater";
+import { desktopText as t } from "./locale";
 
 const CHECK_INTERVAL_MS = 60 * 60 * 1000; // hourly
 
@@ -39,12 +40,12 @@ export function initAutoUpdater(): void {
     log.info("[updater] update downloaded", info.version);
     const opts = {
       type: "info" as const,
-      buttons: ["Restart", "Later"],
+      buttons: [t("restart"), t("later")],
       defaultId: 0,
       cancelId: 1,
-      title: "Update available",
-      message: `Rescript ${info.version} is ready to install.`,
-      detail: "Restart now to apply the update.",
+      title: t("updateTitle"),
+      message: t("updateMessage", { version: info.version }),
+      detail: t("updateDetail"),
     };
     const win = BrowserWindow.getFocusedWindow();
     void (win ? dialog.showMessageBox(win, opts) : dialog.showMessageBox(opts)).then(
diff --git a/hooks/useDesktopMenu.ts b/hooks/useDesktopMenu.ts
index da2dab1..e7aed12 100644
--- a/hooks/useDesktopMenu.ts
+++ b/hooks/useDesktopMenu.ts
@@ -5,6 +5,8 @@ import { isElectron } from "@/lib/platform";
 import { deleteProject, listProjects } from "@/lib/projects";
 import { useEditorStore } from "@/lib/store";
 import type { MenuCommand } from "@/types/rescript-desktop";
+import { useI18n } from "@/components/I18nProvider";
+import { localizeRuntimeMessage } from "@/lib/i18n";
 
 /**
  * Desktop-only bridge for the native File menu.
@@ -23,6 +25,7 @@ import type { MenuCommand } from "@/types/rescript-desktop";
  *   window from scratch, and it lands long before isolation settles.
  */
 export function useDesktopMenu(openFilePicker: () => void, ready: boolean): void {
+  const { t } = useI18n();
   const projectId = useEditorStore((s) => s.projectId);
   const status = useEditorStore((s) => s.status);
 
@@ -50,7 +53,7 @@ export function useDesktopMenu(openFilePicker: () => void, ready: boolean): void
   }, [syncRecents, projectId, status]);
 
   const clearRecents = useCallback(async () => {
-    if (!confirm("Remove all recent projects? Their saved edits are deleted.")) return;
+    if (!confirm(t("confirm.clearRecent"))) return;
     try {
       const projects = await listProjects();
       const active = useEditorStore.getState().projectId;
@@ -60,11 +63,11 @@ export function useDesktopMenu(openFilePicker: () => void, ready: boolean): void
       }
     } catch (err) {
       console.error(err);
-      alert("Could not clear recent projects.");
+      alert(t("error.clearRecent"));
     } finally {
       await syncRecents();
     }
-  }, [syncRecents]);
+  }, [syncRecents, t]);
 
   const closeProject = useCallback(async () => {
     // The window stays; only the project goes. Flush the debounced autosave
@@ -98,7 +101,11 @@ export function useDesktopMenu(openFilePicker: () => void, ready: boolean): void
             .openProject(command.id)
             .catch(async (err) => {
               console.error(err);
-              alert(err instanceof Error ? err.message : "Could not open that project.");
+              alert(
+                err instanceof Error
+                  ? localizeRuntimeMessage(err.message, t)
+                  : t("error.openProject")
+              );
               await syncRecents();
             });
           return;
@@ -109,7 +116,7 @@ export function useDesktopMenu(openFilePicker: () => void, ready: boolean): void
           void closeProject();
       }
     },
-    [clearRecents, closeProject, syncRecents]
+    [clearRecents, closeProject, syncRecents, t]
   );
 
   // Opening a project can't run before the page is cross-origin isolated; hold
diff --git a/hooks/useTranscriber.ts b/hooks/useTranscriber.ts
index a52fd14..5a854ba 100644
--- a/hooks/useTranscriber.ts
+++ b/hooks/useTranscriber.ts
@@ -6,6 +6,7 @@ import { reportError } from "@/lib/sentry";
 import { useEditorStore } from "@/lib/store";
 import { trackEvent } from "@/lib/telemetry";
 import type { WorkerResponse } from "@/lib/types";
+import { resolveTranscriptLanguage } from "@/lib/transcriptionOptions";
 
 let activeWorker: Worker | null = null;
 
@@ -33,7 +34,10 @@ export function useTranscriber() {
       return;
     }
     const model = store.source;
-    const transcriptLanguage = store.transcriptLanguage;
+    const transcriptLanguagePreference = store.transcriptLanguage;
+    const transcriptLanguage = resolveTranscriptLanguage(
+      transcriptLanguagePreference
+    );
     store.setStatus("transcribing");
     store.setProgress({ message: "Loading speech model…", value: null });
 
@@ -64,7 +68,7 @@ export function useTranscriber() {
           // Nothing about the media itself — not its length, not the text.
           trackEvent("transcription_completed", {
             model,
-            language: transcriptLanguage,
+            language: transcriptLanguagePreference,
           });
           break;
         case "error":
@@ -96,7 +100,12 @@ export function useTranscriber() {
     // long recording the copy this replaces was hundreds of megabytes held for
     // the length of the run.
     workerRef.current.postMessage(
-      { audio, duration, model, language: transcriptLanguage },
+      {
+        audio,
+        duration,
+        model,
+        ...(transcriptLanguage ? { language: transcriptLanguage } : {}),
+      },
       [audio.buffer]
     );
   }, []);
diff --git a/lib/i18n.ts b/lib/i18n.ts
new file mode 100644
index 0000000..4f7afef
--- /dev/null
+++ b/lib/i18n.ts
@@ -0,0 +1,550 @@
+export type UiLocale = "en" | "zh-CN";
+export type UiLocalePreference = "system" | UiLocale;
+
+export const DEFAULT_UI_LOCALE_PREFERENCE: UiLocalePreference = "system";
+export const UI_LOCALE_STORAGE_KEY = "rescript.ui-locale";
+
+const en = {
+  "app.title": "Rescript — edit videos like you edit text",
+  "common.auto": "Auto-detect",
+  "common.cancel": "Cancel",
+  "common.close": "Close",
+  "common.delete": "Delete",
+  "common.download": "Download",
+  "common.import": "Import",
+  "common.loading": "Loading",
+  "common.remove": "Remove",
+  "common.restore": "Restore",
+  "common.retry": "Try again",
+  "common.searchOrCreate": "Search or create…",
+  "common.settings": "Settings",
+  "common.system": "System",
+  "common.tools": "Tools",
+  "settings.appearance": "Appearance",
+  "settings.light": "Light",
+  "settings.dark": "Dark",
+  "settings.interfaceLanguage": "Interface language",
+  "settings.transcriptLanguage": "Transcript language",
+  "settings.transcriptLanguageHelp": "Auto-detect keeps the audio's original language. Choose one if detection is inaccurate.",
+  "settings.privacy": "Privacy",
+  "settings.helpImprove": "Help improve the app",
+  "settings.telemetryHelp": "Send anonymous feature usage statistics and crash reports.",
+  "settings.support": "Support / feedback",
+  "settings.reportIssue": "Report an issue",
+  "settings.homepage": "Homepage",
+  "settings.github": "GitHub",
+  "settings.followX": "Follow on X",
+  "language.english": "English",
+  "language.simplifiedChinese": "简体中文",
+  "model.transcriptSource": "Transcript source",
+  "model.language": "Language",
+  "model.transcriptLanguage": "Transcript language",
+  "model.importTranscript": "Import transcript",
+  "model.parakeetAuto": "Parakeet detects the spoken language automatically; this setting is used only for word alignment.",
+  "model.baseDescription": "Faster download and transcription. Good for most clips.",
+  "model.smallDescription": "More accurate on longer or noisier audio. Larger download.",
+  "model.parakeetDescription": "NVIDIA FastConformer — faster on WebGPU, strong EU-language accuracy. Auto-detects language.",
+  "upload.recentProjects": "Recent projects",
+  "upload.removeRecent": "Remove from recent",
+  "upload.dropOrBrowse": "Drop a video or audio file, or {browse}",
+  "upload.browse": "browse",
+  "upload.chooseTranscriptFirst": "Pick a transcript in the menu above, then drop your media",
+  "upload.willUseTranscript": "Will use {name} · MP4, WebM, MOV, MP3, WAV, …",
+  "upload.mediaFormats": "MP4, WebM, MOV, MP3, WAV, M4A, …",
+  "upload.gettingReady": "Getting things ready",
+  "upload.gettingReadyHelp": "Preparing the local media engine…",
+  "upload.unsupported": "This browser cannot run the local media engine",
+  "upload.unsupportedHelp": "Use a current Chromium-based browser or the desktop app.",
+  "upload.transcribeTitle": "Transcribe",
+  "upload.transcribeText": "Whisper locally, or import SRT / VTT.",
+  "upload.editTitle": "Edit",
+  "upload.editText": "Select words and hit delete to edit.",
+  "upload.exportTitle": "Export",
+  "upload.exportText": "Render the final cut to MP4 or M4A.",
+  "upload.justNow": "just now",
+  "editor.chooseTranscript": "Choose a transcript file from the source menu first.",
+  "editor.chooseMedia": "Please choose a video or audio file.",
+  "editor.undo": "Undo (⌘Z)",
+  "editor.redo": "Redo (⇧⌘Z)",
+  "editor.export": "Export",
+  "topbar.startOver": "Start over",
+  "transcript.replace": "Replace transcript from SRT, VTT, or JSON",
+  "transcript.header": "Transcript",
+  "transcript.wordsDeleted": "{count} words",
+  "transcript.replaceConfirm": "Replace the current transcript with this file?",
+  "transcript.invalidFile": "Choose an SRT, VTT, or JSON transcript file.",
+  "transcript.noSpeech": "No speech detected. Make sure this file has audio, or import a transcript.",
+  "transcript.cut": "Cut",
+  "transcript.follow": "Follow playhead",
+  "transcript.hideDeleted": "Hide deleted words",
+  "transcript.showDeleted": "Show deleted words",
+  "transcript.correct": "Correct",
+  "transcript.scrollWithPlayhead": "Scroll with the playhead",
+  "transcript.joinSplit": "Clip split — click to join these clips",
+  "transcript.joinClips": "Join clips",
+  "transcript.hesitation": "Detected hesitation (not transcribed) — cut with Remove filler words",
+  "tools.bulk": "Bulk transcript cleanups",
+  "tools.removeFillers": "Remove filler words",
+  "tools.removeFillersTitle": "Cut filler words (\"um\", \"uh\", \"...\", …) from the media",
+  "tools.restoreFillers": "Restore filler words",
+  "tools.restoreFillersTitle": "Bring every cut filler word back",
+  "tools.removeSilences": "Remove silences",
+  "tools.removeSilencesTitle": "Cut pauses and silences (≥{seconds}s) from the media",
+  "tools.restoreSilences": "Restore silences",
+  "tools.restoreSilencesTitle": "Bring every cut pause back",
+  "timeline.removed": "{trimmed} removed — original length {duration}",
+  "timeline.back5": "Back 5 s",
+  "timeline.playPause": "Play / pause (space)",
+  "timeline.forward5": "Forward 5 s",
+  "timeline.split": "Split",
+  "timeline.splitTitle": "Split clip at playhead (S)",
+  "timeline.splitDisabled": "Move the playhead onto a kept region to split",
+  "timeline.delete": "Delete",
+  "timeline.deleteTitle": "Delete selected clip (Delete)",
+  "timeline.deleteDisabled": "Select a clip on the timeline to delete",
+  "timeline.restore": "Restore",
+  "timeline.restoreTitle": "Restore selected cut (Delete)",
+  "timeline.restoreDisabled": "Select a cut or silence section on the timeline to restore",
+  "timeline.zoomOut": "Zoom out",
+  "timeline.fit": "Fit to window",
+  "timeline.zoomIn": "Zoom in — drag word edges to refine timing",
+  "timeline.joinClips": "Join these clips (remove split)",
+  "timeline.trimStart": "Trim clip start",
+  "timeline.trimEnd": "Trim clip end",
+  "timeline.hesitationAdjust": "… detected hesitation — drag edges to adjust, or Remove filler words to cut",
+  "timeline.hesitationCut": "… detected hesitation — cut with Remove filler words",
+  "timeline.dragTiming": "{word} — drag edges to adjust timing",
+  "timeline.scrollZoom": "Scroll to zoom in/out",
+  "speaker.moveStart": "Drag to move where this speaker starts",
+  "speaker.moveLabel": "Move speaker label",
+  "speaker.options": "Speaker options",
+  "speaker.change": "Change speaker",
+  "speaker.rename": "Rename speaker",
+  "speaker.renameAction": "Rename",
+  "speaker.replace": "Replace in project with…",
+  "speaker.removeProject": "Remove from project",
+  "speaker.newName": "New name",
+  "speaker.find": "Find a speaker…",
+  "speaker.noMatches": "No matching speakers",
+  "speaker.renameTo": "Rename {current} to {next}",
+  "speaker.button": "Speaker",
+  "speaker.search": "Search or create…",
+  "speaker.defaultName": "Speaker {number}",
+  "speaker.create": "Create \"{name}\"",
+  "export.title": "Export",
+  "export.type": "Export type",
+  "export.video": "Video",
+  "export.audio": "Audio",
+  "export.transcript": "Transcript",
+  "export.subtitles": "Subtitles",
+  "export.videoUnavailable": "Video export isn’t available for audio-only projects",
+  "export.noAudio": "This file has no audio track",
+  "export.noWordsFirst": "Transcribe or import a transcript first",
+  "export.format": "Format",
+  "export.resolution": "Resolution",
+  "export.original": "Original",
+  "export.plainText": "Plain text",
+  "export.statOriginal": "Original",
+  "export.statCuts": "Cuts",
+  "export.statEdited": "Edited",
+  "export.transcriptHelp": "Speaker-labeled text with cuts removed. No timestamps.",
+  "export.subtitlesHelp": "SRT and VTT use the edited timeline with cuts applied. JSON keeps the full word list for re-import.",
+  "export.encodingHelp": "Re-encoding on your device — longer files take a while.",
+  "export.reexport": "Re-export with latest edits",
+  "export.rendering": "Rendering on your device…",
+  "export.downloadFile": "Download {name}",
+  "export.downloadFormat": "Download .{format}",
+  "export.exportFormat": "Export {format}",
+  "export.tryAgain": "Try again",
+  "import.reading": "Reading transcript…",
+  "import.importing": "Import…",
+  "import.failed": "Import failed",
+  "import.readingFile": "Reading file…",
+  "import.chooseFileShort": "Choose a file…",
+  "import.chooseFile": "Choose an SRT, VTT, or JSON transcript",
+  "import.selected": "{name} selected",
+  "banner.faster": "Want faster transcription and exports?",
+  "banner.getDesktop": "Get the Rescript desktop app",
+  "banner.downloadFor": "Download for {platform}",
+  "banner.dismiss": "Dismiss",
+  "social.githubRepo": "GitHub repository",
+  "social.discordServer": "Discord server",
+  "social.xProfile": "X profile",
+  "globalError.title": "Rescript — something went wrong",
+  "globalError.heading": "Something went wrong",
+  "globalError.body": "The editor hit an unexpected error. Your saved projects are still on this device — reloading should bring them back.",
+  "progress.loadingMedia": "Loading media…",
+  "progress.loadingMediaEngine": "Loading media engine…",
+  "progress.extractingAudio": "Extracting audio…",
+  "progress.loadingSpeechModel": "Loading speech model…",
+  "progress.loadingSpeechCache": "Loading speech model from cache…",
+  "progress.downloadingSpeech": "Downloading speech model…",
+  "progress.gpuFallback": "GPU interrupted — continuing on CPU…",
+  "progress.detectingSpeech": "Detecting speech…",
+  "progress.detectingLanguage": "Detecting source language…",
+  "progress.transcribing": "Transcribing…",
+  "progress.loadingAlignCache": "Loading alignment model from cache…",
+  "progress.downloadingAlign": "Downloading alignment model…",
+  "progress.aligning": "Aligning words…",
+  "progress.speakers": "Identifying speakers…",
+  "error.selectModel": "Select a speech model to transcribe.",
+  "error.workerCrashed": "Transcription worker crashed.",
+  "error.mediaEngineNetwork": "Couldn't load the media engine — the connection dropped. Check your internet and try again.",
+  "error.processFile": "Failed to process this file.",
+  "error.extractAudio": "Could not extract audio from this file.",
+  "error.nothingToExport": "Everything has been deleted — nothing to export.",
+  "error.videoExport": "Export failed while rendering the video.",
+  "error.audioExport": "Export failed while rendering the audio.",
+  "error.export": "Export failed.",
+  "error.emptyTranscript": "That transcript file is empty.",
+  "error.noTimedWords": "No timed words found in that transcript.",
+  "error.parseJson": "Could not parse that JSON transcript.",
+  "error.jsonShape": "JSON must be a word array or an object containing words.",
+  "error.noWords": "No words to export.",
+  "error.projectMissing": "That project is no longer saved.",
+  "error.openProject": "Could not open that project.",
+  "error.removeProject": "Could not remove that project.",
+  "error.readTranscript": "Could not read that transcript.",
+  "error.clearRecent": "Could not clear recent projects.",
+  "error.modelDownload": "Couldn't finish downloading the speech model — the connection dropped. Check your internet and try again; completed parts are kept.",
+  "error.gpuReset": "Transcription was interrupted when the GPU reset, often after locking the screen. Please try again.",
+  "confirm.clearRecent": "Remove all recent projects? Their saved edits are deleted.",
+} as const;
+
+export type MessageKey = keyof typeof en;
+
+const zhCN: Record<MessageKey, string> = {
+  "app.title": "Rescript — 像编辑文字一样编辑视频",
+  "common.auto": "自动检测",
+  "common.cancel": "取消",
+  "common.close": "关闭",
+  "common.delete": "删除",
+  "common.download": "下载",
+  "common.import": "导入",
+  "common.loading": "正在加载",
+  "common.remove": "移除",
+  "common.restore": "恢复",
+  "common.retry": "重试",
+  "common.searchOrCreate": "搜索或新建…",
+  "common.settings": "设置",
+  "common.system": "跟随系统",
+  "common.tools": "工具",
+  "settings.appearance": "外观",
+  "settings.light": "浅色",
+  "settings.dark": "深色",
+  "settings.interfaceLanguage": "界面语言",
+  "settings.transcriptLanguage": "转录语言",
+  "settings.transcriptLanguageHelp": "自动检测会保留音频的原语言；检测不准时可手动指定。",
+  "settings.privacy": "隐私",
+  "settings.helpImprove": "帮助改进应用",
+  "settings.telemetryHelp": "发送匿名功能使用统计和崩溃报告。",
+  "settings.support": "支持与反馈",
+  "settings.reportIssue": "报告问题",
+  "settings.homepage": "主页",
+  "settings.github": "GitHub",
+  "settings.followX": "在 X 上关注",
+  "language.english": "English",
+  "language.simplifiedChinese": "简体中文",
+  "model.transcriptSource": "转录来源",
+  "model.language": "语言",
+  "model.transcriptLanguage": "转录语言",
+  "model.importTranscript": "导入转录文本",
+  "model.parakeetAuto": "Parakeet 会自动识别语音语言；此设置只用于词语对齐。",
+  "model.baseDescription": "下载和转录更快，适合大多数片段。",
+  "model.smallDescription": "较长或噪声较多的音频更准确，但下载体积更大。",
+  "model.parakeetDescription": "NVIDIA FastConformer；WebGPU 下更快，擅长欧洲语言并自动识别语种。",
+  "upload.recentProjects": "最近项目",
+  "upload.removeRecent": "从最近项目中移除",
+  "upload.dropOrBrowse": "拖入视频或音频文件，或者{browse}",
+  "upload.browse": "浏览文件",
+  "upload.chooseTranscriptFirst": "请先在上方菜单选择转录文本，再拖入媒体文件",
+  "upload.willUseTranscript": "将使用 {name} · MP4、WebM、MOV、MP3、WAV 等",
+  "upload.mediaFormats": "MP4、WebM、MOV、MP3、WAV、M4A 等",
+  "upload.gettingReady": "正在准备",
+  "upload.gettingReadyHelp": "正在准备本地媒体引擎…",
+  "upload.unsupported": "此浏览器无法运行本地媒体引擎",
+  "upload.unsupportedHelp": "请使用新版 Chromium 浏览器或桌面客户端。",
+  "upload.transcribeTitle": "转录",
+  "upload.transcribeText": "在本机运行 Whisper，或导入 SRT / VTT。",
+  "upload.editTitle": "编辑",
+  "upload.editText": "选择文字并按删除键即可剪除内容。",
+  "upload.exportTitle": "导出",
+  "upload.exportText": "将最终成片导出为 MP4 或 M4A。",
+  "upload.justNow": "刚刚",
+  "editor.chooseTranscript": "请先从转录来源菜单选择一个转录文本文件。",
+  "editor.chooseMedia": "请选择视频或音频文件。",
+  "editor.undo": "撤销 (⌘Z)",
+  "editor.redo": "重做 (⇧⌘Z)",
+  "editor.export": "导出",
+  "topbar.startOver": "重新开始",
+  "transcript.replace": "用 SRT、VTT 或 JSON 替换转录文本",
+  "transcript.header": "转录文本",
+  "transcript.wordsDeleted": "已删除 {count} 个词",
+  "transcript.replaceConfirm": "要用此文件替换当前转录文本吗？",
+  "transcript.invalidFile": "请选择 SRT、VTT 或 JSON 转录文本文件。",
+  "transcript.noSpeech": "未检测到语音。请确认文件包含音轨，或导入转录文本。",
+  "transcript.cut": "剪除",
+  "transcript.follow": "跟随播放位置",
+  "transcript.hideDeleted": "隐藏已删除文字",
+  "transcript.showDeleted": "显示已删除文字",
+  "transcript.correct": "纠正",
+  "transcript.scrollWithPlayhead": "跟随播放位置滚动",
+  "transcript.joinSplit": "片段分割点 — 点击以合并片段",
+  "transcript.joinClips": "合并片段",
+  "transcript.hesitation": "检测到未转录的停顿语气，可用“移除语气词”剪除",
+  "tools.bulk": "批量清理转录文本",
+  "tools.removeFillers": "移除语气词",
+  "tools.removeFillersTitle": "从媒体中剪除“嗯”“呃”“…”等语气词",
+  "tools.restoreFillers": "恢复语气词",
+  "tools.restoreFillersTitle": "恢复所有已剪除的语气词",
+  "tools.removeSilences": "移除静音",
+  "tools.removeSilencesTitle": "剪除不少于 {seconds} 秒的停顿和静音",
+  "tools.restoreSilences": "恢复静音",
+  "tools.restoreSilencesTitle": "恢复所有已剪除的停顿",
+  "timeline.removed": "已剪除 {trimmed} — 原始长度 {duration}",
+  "timeline.back5": "后退 5 秒",
+  "timeline.playPause": "播放 / 暂停（空格）",
+  "timeline.forward5": "前进 5 秒",
+  "timeline.split": "分割",
+  "timeline.splitTitle": "在播放位置分割片段（S）",
+  "timeline.splitDisabled": "请把播放位置移到保留区域后再分割",
+  "timeline.delete": "删除",
+  "timeline.deleteTitle": "删除所选片段（Delete）",
+  "timeline.deleteDisabled": "请先在时间轴上选择要删除的片段",
+  "timeline.restore": "恢复",
+  "timeline.restoreTitle": "恢复所选剪除区域（Delete）",
+  "timeline.restoreDisabled": "请先选择要恢复的剪除或静音区域",
+  "timeline.zoomOut": "缩小",
+  "timeline.fit": "适合窗口",
+  "timeline.zoomIn": "放大 — 拖动文字边缘可微调时间",
+  "timeline.joinClips": "合并片段（移除分割点）",
+  "timeline.trimStart": "修剪片段开头",
+  "timeline.trimEnd": "修剪片段结尾",
+  "timeline.hesitationAdjust": "检测到停顿语气 — 可拖动边缘调整，或用“移除语气词”剪除",
+  "timeline.hesitationCut": "检测到停顿语气 — 可用“移除语气词”剪除",
+  "timeline.dragTiming": "{word} — 拖动边缘可调整时间",
+  "timeline.scrollZoom": "滚动以缩放时间轴",
+  "speaker.moveStart": "拖动以调整此说话人的起始位置",
+  "speaker.moveLabel": "移动说话人标签",
+  "speaker.options": "说话人选项",
+  "speaker.change": "更改说话人",
+  "speaker.rename": "重命名说话人",
+  "speaker.renameAction": "重命名",
+  "speaker.replace": "在整个项目中替换为…",
+  "speaker.removeProject": "从项目中移除",
+  "speaker.newName": "新名称",
+  "speaker.find": "查找说话人…",
+  "speaker.noMatches": "没有匹配的说话人",
+  "speaker.renameTo": "将 {current} 重命名为 {next}",
+  "speaker.button": "说话人",
+  "speaker.search": "搜索或新建…",
+  "speaker.defaultName": "说话人 {number}",
+  "speaker.create": "新建“{name}”",
+  "export.title": "导出",
+  "export.type": "导出类型",
+  "export.video": "视频",
+  "export.audio": "音频",
+  "export.transcript": "转录文本",
+  "export.subtitles": "字幕",
+  "export.videoUnavailable": "纯音频项目不能导出视频",
+  "export.noAudio": "此文件没有音轨",
+  "export.noWordsFirst": "请先转录或导入转录文本",
+  "export.format": "格式",
+  "export.resolution": "分辨率",
+  "export.original": "原始",
+  "export.plainText": "纯文本",
+  "export.statOriginal": "原始",
+  "export.statCuts": "剪除",
+  "export.statEdited": "成片",
+  "export.transcriptHelp": "带说话人标签的文本，已移除剪除内容，不含时间戳。",
+  "export.subtitlesHelp": "SRT 和 VTT 使用已应用剪除的成片时间轴；JSON 保留完整词表以便重新导入。",
+  "export.encodingHelp": "正在本机重新编码，较长文件需要更多时间。",
+  "export.reexport": "按最新编辑重新导出",
+  "export.rendering": "正在本机渲染…",
+  "export.downloadFile": "下载 {name}",
+  "export.downloadFormat": "下载 .{format}",
+  "export.exportFormat": "导出 {format}",
+  "export.tryAgain": "重新导出",
+  "import.reading": "正在读取转录文本…",
+  "import.importing": "正在导入…",
+  "import.failed": "导入失败",
+  "import.readingFile": "正在读取文件…",
+  "import.chooseFileShort": "请选择文件…",
+  "import.chooseFile": "选择 SRT、VTT 或 JSON 转录文本",
+  "import.selected": "已选择 {name}",
+  "banner.faster": "想要更快地转录和导出？",
+  "banner.getDesktop": "获取 Rescript 桌面客户端",
+  "banner.downloadFor": "下载 {platform} 版",
+  "banner.dismiss": "关闭",
+  "social.githubRepo": "GitHub 仓库",
+  "social.discordServer": "Discord 服务器",
+  "social.xProfile": "X 个人主页",
+  "globalError.title": "Rescript — 出现错误",
+  "globalError.heading": "出现错误",
+  "globalError.body": "编辑器遇到了意外错误。已保存的项目仍在此设备上，重新加载后即可恢复。",
+  "progress.loadingMedia": "正在加载媒体…",
+  "progress.loadingMediaEngine": "正在加载媒体引擎…",
+  "progress.extractingAudio": "正在提取音频…",
+  "progress.loadingSpeechModel": "正在加载语音模型…",
+  "progress.loadingSpeechCache": "正在从缓存加载语音模型…",
+  "progress.downloadingSpeech": "正在下载语音模型…",
+  "progress.gpuFallback": "GPU 已中断，正在改用 CPU…",
+  "progress.detectingSpeech": "正在检测语音…",
+  "progress.detectingLanguage": "正在识别源语言…",
+  "progress.transcribing": "正在转录…",
+  "progress.loadingAlignCache": "正在从缓存加载对齐模型…",
+  "progress.downloadingAlign": "正在下载对齐模型…",
+  "progress.aligning": "正在对齐文字…",
+  "progress.speakers": "正在识别说话人…",
+  "error.selectModel": "请选择用于转录的语音模型。",
+  "error.workerCrashed": "转录进程发生崩溃。",
+  "error.mediaEngineNetwork": "媒体引擎加载失败，网络连接已中断。请检查网络后重试。",
+  "error.processFile": "无法处理此文件。",
+  "error.extractAudio": "无法从此文件提取音频。",
+  "error.nothingToExport": "所有内容都已删除，没有可导出的内容。",
+  "error.videoExport": "渲染视频时导出失败。",
+  "error.audioExport": "渲染音频时导出失败。",
+  "error.export": "导出失败。",
+  "error.emptyTranscript": "转录文本文件为空。",
+  "error.noTimedWords": "转录文本中没有带时间信息的文字。",
+  "error.parseJson": "无法解析 JSON 转录文本。",
+  "error.jsonShape": "JSON 必须是文字数组，或包含 words 字段的对象。",
+  "error.noWords": "没有可导出的文字。",
+  "error.projectMissing": "此项目已不在已保存项目中。",
+  "error.openProject": "无法打开此项目。",
+  "error.removeProject": "无法移除此项目。",
+  "error.readTranscript": "无法读取此转录文本。",
+  "error.clearRecent": "无法清除最近项目。",
+  "error.modelDownload": "语音模型下载未完成，网络连接已中断。请检查网络后重试；已下载完成的部分会被保留。",
+  "error.gpuReset": "GPU 重置导致转录中断（常见于锁屏后），请重试。",
+  "confirm.clearRecent": "要移除所有最近项目吗？已保存的编辑也会被删除。",
+};
+
+export type Translate = (
+  key: MessageKey,
+  params?: Record<string, string | number>
+) => string;
+
+export function isUiLocalePreference(value: unknown): value is UiLocalePreference {
+  return value === "system" || value === "en" || value === "zh-CN";
+}
+
+export function resolveUiLocale(
+  preference: UiLocalePreference,
+  systemLanguages: readonly string[]
+): UiLocale {
+  if (preference !== "system") return preference;
+  for (const raw of systemLanguages) {
+    const language = raw.toLowerCase();
+    if (language === "zh" || language.startsWith("zh-")) return "zh-CN";
+    if (language === "en" || language.startsWith("en-")) return "en";
+  }
+  return "en";
+}
+
+export function systemLanguages(): string[] {
+  if (typeof navigator === "undefined") return [];
+  return navigator.languages?.length
+    ? Array.from(navigator.languages)
+    : navigator.language
+      ? [navigator.language]
+      : [];
+}
+
+export function loadUiLocalePreference(): UiLocalePreference {
+  if (typeof window === "undefined") return DEFAULT_UI_LOCALE_PREFERENCE;
+  try {
+    const value = window.localStorage.getItem(UI_LOCALE_STORAGE_KEY);
+    if (isUiLocalePreference(value)) return value;
+  } catch {
+    // Private mode / disabled storage.
+  }
+  return DEFAULT_UI_LOCALE_PREFERENCE;
+}
+
+export function saveUiLocalePreference(preference: UiLocalePreference): void {
+  if (typeof window === "undefined") return;
+  try {
+    window.localStorage.setItem(UI_LOCALE_STORAGE_KEY, preference);
+  } catch {
+    // Private mode / disabled storage.
+  }
+}
+
+export function translate(
+  locale: UiLocale,
+  key: MessageKey,
+  params: Record<string, string | number> = {}
+): string {
+  const template = (locale === "zh-CN" ? zhCN : en)[key] ?? en[key];
+  return template.replace(/\{(\w+)\}/g, (token, name: string) =>
+    Object.prototype.hasOwnProperty.call(params, name) ? String(params[name]) : token
+  );
+}
+
+const runtimeMessageKeys: Record<string, MessageKey> = {
+  "Loading media…": "progress.loadingMedia",
+  "Loading media engine…": "progress.loadingMediaEngine",
+  "Extracting audio…": "progress.extractingAudio",
+  "Loading speech model…": "progress.loadingSpeechModel",
+  "Loading speech model from cache…": "progress.loadingSpeechCache",
+  "Downloading speech model…": "progress.downloadingSpeech",
+  "GPU interrupted — continuing on CPU…": "progress.gpuFallback",
+  "Detecting speech…": "progress.detectingSpeech",
+  "Detecting language…": "progress.detectingLanguage",
+  "Transcribing…": "progress.transcribing",
+  "Loading alignment model from cache…": "progress.loadingAlignCache",
+  "Downloading alignment model…": "progress.downloadingAlign",
+  "Aligning words…": "progress.aligning",
+  "Identifying speakers…": "progress.speakers",
+  "Select a speech model to transcribe.": "error.selectModel",
+  "Transcription worker crashed.": "error.workerCrashed",
+  "Couldn't load the media engine — the connection dropped. Check your internet and try again.": "error.mediaEngineNetwork",
+  "Failed to process this file.": "error.processFile",
+  "Could not extract audio from this file.": "error.extractAudio",
+  "Everything has been deleted — nothing to export.": "error.nothingToExport",
+  "Export failed while rendering the video.": "error.videoExport",
+  "Export failed while rendering the audio.": "error.audioExport",
+  "Export failed.": "error.export",
+  "That transcript file is empty.": "error.emptyTranscript",
+  "No timed words found in that transcript.": "error.noTimedWords",
+  "Could not parse that JSON transcript.": "error.parseJson",
+  'JSON must be a word array or { "words": [...] }.': "error.jsonShape",
+  "No words to export.": "error.noWords",
+  "That project is no longer saved.": "error.projectMissing",
+  "Could not open that project.": "error.openProject",
+  "Could not remove that project.": "error.removeProject",
+  "Could not read that transcript.": "error.readTranscript",
+  "Could not clear recent projects.": "error.clearRecent",
+  "Couldn't finish downloading the speech model — the connection dropped. Check your internet and try again; the parts that finished downloading are kept.": "error.modelDownload",
+  "Transcription was interrupted when the GPU reset (often after locking the screen). Please try again.": "error.gpuReset",
+};
+
+export function localizeRuntimeMessage(
+  text: string | null | undefined,
+  t: Translate
+): string {
+  if (!text) return "";
+  const key = runtimeMessageKeys[text];
+  return key ? t(key) : text;
+}
+
+export function formatRelativeTime(
+  locale: UiLocale,
+  timestamp: number,
+  now = Date.now()
+): string {
+  const seconds = Math.max(0, Math.round((now - timestamp) / 1000));
+  const formatter = new Intl.RelativeTimeFormat(locale, { numeric: "auto" });
+  if (seconds < 45) return formatter.format(0, "second");
+  const minutes = Math.round(seconds / 60);
+  if (minutes < 60) return formatter.format(-minutes, "minute");
+  const hours = Math.round(minutes / 60);
+  if (hours < 48) return formatter.format(-hours, "hour");
+  const days = Math.round(hours / 24);
+  if (days < 14) return formatter.format(-days, "day");
+  return new Intl.DateTimeFormat(locale, {
+    month: "short",
+    day: "numeric",
+  }).format(timestamp);
+}
diff --git a/lib/languages.ts b/lib/languages.ts
index 0074b66..2fb3743 100644
--- a/lib/languages.ts
+++ b/lib/languages.ts
@@ -1,4 +1,5 @@
 export type TranscriptLanguage = "en" | "es" | "fr" | "de" | "zh";
+export type TranscriptLanguagePreference = "auto" | TranscriptLanguage;
 
 export interface TranscriptLanguageInfo {
   label: string;
@@ -11,7 +12,14 @@ export interface TranscriptLanguageInfo {
 
 const LANGUAGE_STORAGE_KEY = "rescript.transcript-language";
 
-export const DEFAULT_TRANSCRIPT_LANGUAGE: TranscriptLanguage = "en";
+export const DEFAULT_TRANSCRIPT_LANGUAGE: TranscriptLanguagePreference = "auto";
+
+export const AUTO_TRANSCRIPT_LANGUAGE_INFO = {
+  label: "Auto-detect",
+  nativeLabel: "Auto-detect",
+  flag: "🌐",
+  code: "AUTO",
+} satisfies TranscriptLanguageInfo;
 
 export const TRANSCRIPT_LANGUAGES: Record<
   TranscriptLanguage,
@@ -57,6 +65,11 @@ export const TRANSCRIPT_LANGUAGE_ORDER: TranscriptLanguage[] = [
   "zh",
 ];
 
+export const TRANSCRIPT_LANGUAGE_PREFERENCE_ORDER: TranscriptLanguagePreference[] = [
+  "auto",
+  ...TRANSCRIPT_LANGUAGE_ORDER,
+];
+
 export function isTranscriptLanguage(
   value: unknown
 ): value is TranscriptLanguage {
@@ -69,12 +82,18 @@ export function isTranscriptLanguage(
   );
 }
 
+export function isTranscriptLanguagePreference(
+  value: unknown
+): value is TranscriptLanguagePreference {
+  return value === "auto" || isTranscriptLanguage(value);
+}
+
 /** Read the last-selected transcript language from localStorage. */
-export function loadTranscriptLanguagePreference(): TranscriptLanguage {
+export function loadTranscriptLanguagePreference(): TranscriptLanguagePreference {
   if (typeof window === "undefined") return DEFAULT_TRANSCRIPT_LANGUAGE;
   try {
     const raw = window.localStorage.getItem(LANGUAGE_STORAGE_KEY);
-    if (isTranscriptLanguage(raw)) return raw;
+    if (isTranscriptLanguagePreference(raw)) return raw;
   } catch {
     // private mode / disabled storage
   }
@@ -82,7 +101,9 @@ export function loadTranscriptLanguagePreference(): TranscriptLanguage {
 }
 
 /** Persist the selected transcript language for the next visit. */
-export function saveTranscriptLanguagePreference(language: TranscriptLanguage) {
+export function saveTranscriptLanguagePreference(
+  language: TranscriptLanguagePreference
+) {
   if (typeof window === "undefined") return;
   try {
     window.localStorage.setItem(LANGUAGE_STORAGE_KEY, language);
diff --git a/lib/projects.ts b/lib/projects.ts
index f4e2719..c539d72 100644
--- a/lib/projects.ts
+++ b/lib/projects.ts
@@ -7,10 +7,10 @@
  */
 
 import { isTranscriptSource, type TranscriptSource } from "./source";
-import type { TranscriptLanguage } from "./languages";
+import type { TranscriptLanguagePreference } from "./languages";
 import {
   DEFAULT_TRANSCRIPT_LANGUAGE,
-  isTranscriptLanguage,
+  isTranscriptLanguagePreference,
 } from "./languages";
 import type { MediaKind } from "./media";
 import type { ManualCut, SceneBoundary, SpeakerInfo, Word } from "./types";
@@ -26,7 +26,7 @@ export interface ProjectMeta {
   mediaKind: MediaKind;
   duration: number;
   source: TranscriptSource;
-  transcriptLanguage: TranscriptLanguage;
+  transcriptLanguage: TranscriptLanguagePreference;
   updatedAt: number;
   createdAt: number;
 }
@@ -137,7 +137,7 @@ export async function listProjects(): Promise<ProjectMeta[]> {
       mediaKind: r.mediaKind,
       duration: r.duration,
       source: projectSource(r),
-      transcriptLanguage: isTranscriptLanguage(r.transcriptLanguage)
+      transcriptLanguage: isTranscriptLanguagePreference(r.transcriptLanguage)
         ? r.transcriptLanguage
         : DEFAULT_TRANSCRIPT_LANGUAGE,
       updatedAt: r.updatedAt,
@@ -181,7 +181,7 @@ export async function putProject(input: ProjectWrite): Promise<string> {
     mediaKind: input.mediaKind,
     duration: input.duration,
     source: isTranscriptSource(input.source) ? input.source : "base",
-    transcriptLanguage: isTranscriptLanguage(input.transcriptLanguage)
+    transcriptLanguage: isTranscriptLanguagePreference(input.transcriptLanguage)
       ? input.transcriptLanguage
       : DEFAULT_TRANSCRIPT_LANGUAGE,
     words: input.words,
diff --git a/lib/store.ts b/lib/store.ts
index 4c7e867..77c5d21 100644
--- a/lib/store.ts
+++ b/lib/store.ts
@@ -31,10 +31,10 @@ import { isTranscriptSource, type TranscriptSource } from "./source";
 import { trackEvent } from "./telemetry";
 import {
   DEFAULT_TRANSCRIPT_LANGUAGE,
-  isTranscriptLanguage,
+  isTranscriptLanguagePreference,
   loadTranscriptLanguagePreference,
   saveTranscriptLanguagePreference,
-  type TranscriptLanguage,
+  type TranscriptLanguagePreference,
 } from "./languages";
 import { detectMediaKind, type MediaKind } from "./media";
 import { buildWaveformPeaks, type WaveformPeaks } from "./waveform";
@@ -80,7 +80,7 @@ interface EditorState {
   /** Transcript source selected on the upload screen (speech model or import). */
   source: TranscriptSource;
   /** Language hint sent to Whisper when transcribing (Parakeet auto-detects). */
-  transcriptLanguage: TranscriptLanguage;
+  transcriptLanguage: TranscriptLanguagePreference;
   /**
    * Caption file parsed on the upload screen when source is "import".
    * Cleared when switching back to a speech model or after media loads.
@@ -150,7 +150,7 @@ interface EditorState {
   /** Delete a saved project; if it is the active one, resets to the home screen. */
   removeProject: (id: string) => Promise<void>;
   setSource: (s: TranscriptSource) => void;
-  setTranscriptLanguage: (language: TranscriptLanguage) => void;
+  setTranscriptLanguage: (language: TranscriptLanguagePreference) => void;
   setPendingTranscript: (t: PendingTranscript | null) => void;
   setDuration: (d: number) => void;
   /**
@@ -428,7 +428,7 @@ export const useEditorStore = create<EditorState>((set, get) => ({
       mediaKind: record.mediaKind,
       duration: record.duration,
       source: isTranscriptSource(record.source) ? record.source : "base",
-      transcriptLanguage: isTranscriptLanguage(record.transcriptLanguage)
+      transcriptLanguage: isTranscriptLanguagePreference(record.transcriptLanguage)
         ? record.transcriptLanguage
         : DEFAULT_TRANSCRIPT_LANGUAGE,
       projectId: record.id,
diff --git a/lib/transcriptionOptions.ts b/lib/transcriptionOptions.ts
new file mode 100644
index 0000000..24a0c4d
--- /dev/null
+++ b/lib/transcriptionOptions.ts
@@ -0,0 +1,28 @@
+import type {
+  TranscriptLanguage,
+  TranscriptLanguagePreference,
+} from "./languages";
+
+export interface WhisperLanguageOptions {
+  task: "transcribe";
+  language?: string;
+}
+
+/** Resolve a saved UI preference to the concrete hint accepted by ASR backends. */
+export function resolveTranscriptLanguage(
+  preference: TranscriptLanguagePreference
+): TranscriptLanguage | undefined {
+  return preference === "auto" ? undefined : preference;
+}
+
+/**
+ * Whisper must always transcribe in the source language. In automatic mode the
+ * language key is omitted so Whisper can detect it from the audio.
+ */
+export function whisperLanguageOptions(
+  language: string | undefined
+): WhisperLanguageOptions {
+  return language
+    ? { task: "transcribe", language }
+    : { task: "transcribe" };
+}
diff --git a/lib/types.ts b/lib/types.ts
index cca974f..9beb9cd 100644
--- a/lib/types.ts
+++ b/lib/types.ts
@@ -103,5 +103,5 @@ export interface WorkerRequest {
    * Transcript language: conditions Whisper decoding, selects the CTC aligner,
    * and is passed through for Parakeet alignment (Parakeet ASR still auto-detects).
    */
-  language: import("./languages").TranscriptLanguage;
+  language?: import("./languages").TranscriptLanguage;
 }
diff --git a/package.json b/package.json
index 1c5dfc9..871527c 100644
--- a/package.json
+++ b/package.json
@@ -17,14 +17,18 @@
   "scripts": {
     "dev": "next dev",
     "build": "next build",
+    "prebuild": "node scripts/copy-assets.mjs",
     "start": "next start",
     "lint": "eslint",
+    "test:localization": "tsx tests/languages-test.ts && tsx tests/transcription-options-test.ts && tsx tests/i18n-test.ts",
     "postinstall": "patch-package && node scripts/copy-assets.mjs",
     "build:electron": "node scripts/build-electron.mjs",
     "typecheck:electron": "tsc -p electron/tsconfig.json --noEmit",
     "electron:dev": "npm run build:electron && concurrently -k -n next,electron -c cyan,magenta \"next dev\" \"node scripts/electron-dev.mjs\"",
     "export:web": "cross-env STATIC_EXPORT=1 next build",
+    "preexport:web": "node scripts/copy-assets.mjs",
     "export:desktop": "cross-env STATIC_EXPORT=1 NEXT_PUBLIC_ELECTRON=1 next build",
+    "preexport:desktop": "node scripts/copy-assets.mjs",
     "dist": "npm run export:desktop && npm run build:electron && electron-builder --config.npmRebuild=false",
     "release": "npm run export:desktop && npm run build:electron && electron-builder --publish always --config.npmRebuild=false",
     "cut:patch": "npm version patch -m \"Release v%s\" && git push --follow-tags",
@@ -133,6 +137,11 @@
       "oneClick": false,
       "perMachine": false,
       "allowToChangeInstallationDirectory": true,
+      "installerLanguages": [
+        "en_US",
+        "zh_CN"
+      ],
+      "multiLanguageInstaller": true,
       "artifactName": "${productName}-Setup.${ext}"
     },
     "linux": {
diff --git a/tests/electron-cdp-e2e.mjs b/tests/electron-cdp-e2e.mjs
new file mode 100644
index 0000000..032677a
--- /dev/null
+++ b/tests/electron-cdp-e2e.mjs
@@ -0,0 +1,252 @@
+import { writeFile } from "node:fs/promises";
+
+const endpoint = process.env.RESCRIPT_CDP_URL ?? "http://127.0.0.1:9223";
+const [command = "inspect", argument] = process.argv.slice(2);
+
+class CdpClient {
+  constructor(socket) {
+    this.socket = socket;
+    this.nextId = 1;
+    this.pending = new Map();
+    socket.addEventListener("message", (event) => {
+      const message = JSON.parse(String(event.data));
+      if (!message.id) return;
+      const pending = this.pending.get(message.id);
+      if (!pending) return;
+      this.pending.delete(message.id);
+      if (message.error) pending.reject(new Error(JSON.stringify(message.error)));
+      else pending.resolve(message.result);
+    });
+  }
+
+  static async connect(url) {
+    const socket = new WebSocket(url);
+    await new Promise((resolve, reject) => {
+      socket.addEventListener("open", resolve, { once: true });
+      socket.addEventListener("error", reject, { once: true });
+    });
+    return new CdpClient(socket);
+  }
+
+  send(method, params = {}) {
+    const id = this.nextId++;
+    return new Promise((resolve, reject) => {
+      this.pending.set(id, { resolve, reject });
+      this.socket.send(JSON.stringify({ id, method, params }));
+    });
+  }
+
+  async evaluate(expression) {
+    const result = await this.send("Runtime.evaluate", {
+      expression,
+      awaitPromise: true,
+      returnByValue: true,
+      userGesture: true,
+    });
+    if (result.exceptionDetails) {
+      throw new Error(result.exceptionDetails.exception?.description ?? "Runtime.evaluate failed");
+    }
+    return result.result.value;
+  }
+
+  close() {
+    this.socket.close();
+  }
+}
+
+const sleep = (ms) => new Promise((resolve) => setTimeout(resolve, ms));
+
+async function waitFor(client, expression, timeoutMs = 15_000) {
+  const deadline = Date.now() + timeoutMs;
+  while (Date.now() < deadline) {
+    const value = await client.evaluate(expression);
+    if (value) return value;
+    await sleep(250);
+  }
+  throw new Error(`Timed out waiting for: ${expression}`);
+}
+
+const pages = await fetch(`${endpoint}/json/list`).then((response) => response.json());
+const page = pages.find((candidate) => candidate.type === "page" && candidate.url.startsWith("app://"));
+if (!page) throw new Error(`No Rescript page at ${endpoint}`);
+
+const client = await CdpClient.connect(page.webSocketDebuggerUrl);
+await client.send("Runtime.enable");
+
+async function inspect() {
+  return client.evaluate(`(() => ({
+    title: document.title,
+    htmlLang: document.documentElement.lang,
+    crossOriginIsolated,
+    inputReady: Boolean(document.querySelector('label input[type="file"]:not([disabled])')),
+    uiLocale: localStorage.getItem('rescript.ui-locale'),
+    transcriptLanguage: localStorage.getItem('rescript.transcript-language'),
+    bodyText: document.body.innerText.slice(0, 8000)
+  }))()`);
+}
+
+async function openSettings() {
+  const opened = await client.evaluate(`(() => {
+    if (document.querySelector('[role="dialog"] select')) return true;
+    const button = [...document.querySelectorAll('button')].find((item) =>
+      ['Settings', '设置'].includes(item.getAttribute('aria-label'))
+    );
+    if (!button) return false;
+    button.click();
+    return true;
+  })()`);
+  if (!opened) throw new Error("Settings button was not found");
+  await waitFor(client, `Boolean(document.querySelector('[role="dialog"] select'))`);
+}
+
+async function changeSelect(optionValue) {
+  await openSettings();
+  const changed = await client.evaluate(`(() => {
+    const select = [...document.querySelectorAll('[role="dialog"] select')].find((item) =>
+      [...item.options].some((option) => option.value === ${JSON.stringify(optionValue)})
+    );
+    if (!select) return false;
+    const valueSetter = Object.getOwnPropertyDescriptor(HTMLSelectElement.prototype, 'value').set;
+    valueSetter.call(select, ${JSON.stringify(optionValue)});
+    select.dispatchEvent(new Event('change', { bubbles: true }));
+    return true;
+  })()`);
+  if (!changed) throw new Error(`No settings select supports ${optionValue}`);
+}
+
+if (command === "inspect") {
+  console.log(JSON.stringify(await inspect(), null, 2));
+} else if (command === "locale") {
+  if (argument !== "en" && argument !== "zh-CN" && argument !== "system") {
+    throw new Error("locale expects en, zh-CN, or system");
+  }
+  await changeSelect(argument);
+  if (argument !== "system") {
+    await waitFor(client, `document.documentElement.lang === ${JSON.stringify(argument)}`);
+  }
+  console.log(JSON.stringify(await inspect(), null, 2));
+} else if (command === "language") {
+  if (!argument) throw new Error("language expects a preference such as auto or zh");
+  await changeSelect(argument);
+  await waitFor(
+    client,
+    `localStorage.getItem('rescript.transcript-language') === ${JSON.stringify(argument)}`
+  );
+  console.log(JSON.stringify(await inspect(), null, 2));
+} else if (command === "upload") {
+  if (!argument) throw new Error("upload expects an absolute media path");
+  await waitFor(client, `Boolean(document.querySelector('label input[type="file"]:not([disabled])'))`);
+  const documentNode = await client.send("DOM.getDocument", { depth: -1, pierce: true });
+  const inputNode = await client.send("DOM.querySelector", {
+    nodeId: documentNode.root.nodeId,
+    selector: 'label input[type="file"]',
+  });
+  if (!inputNode.nodeId) throw new Error("Upload input was not found");
+  await client.send("DOM.setFileInputFiles", { nodeId: inputNode.nodeId, files: [argument] });
+  await client.evaluate(`(() => {
+    const input = document.querySelector('label input[type="file"]');
+    if (!input) return null;
+    input.dispatchEvent(new Event('input', { bubbles: true }));
+    input.dispatchEvent(new Event('change', { bubbles: true }));
+    return input.files?.[0]?.name ?? null;
+  })()`);
+  await waitFor(client, `!document.querySelector('label input[type="file"]')`, 30_000);
+  console.log(JSON.stringify(await inspect(), null, 2));
+} else if (command === "status") {
+  const status = await client.evaluate(`(() => {
+    const words = [...document.querySelectorAll('.transcript-words [data-wid]')]
+      .map((node) => node.textContent.trim())
+      .filter(Boolean);
+    const transcript = words.join('');
+    const exportButton = [...document.querySelectorAll('button')].find((button) =>
+      ['Export', '导出'].includes(button.textContent.trim())
+    );
+    return {
+      htmlLang: document.documentElement.lang,
+      ready: Boolean(exportButton && !exportButton.disabled && words.length),
+      wordCount: words.length,
+      cjkCount: (transcript.match(/[\\u3400-\\u9fff]/g) ?? []).length,
+      englishWordCount: (transcript.match(/[A-Za-z]+/g) ?? []).length,
+      transcript,
+      bodyText: document.body.innerText.slice(0, 8000)
+    };
+  })()`);
+  console.log(JSON.stringify(status, null, 2));
+} else if (command === "diagnostics") {
+  const diagnostics = await client.evaluate(`(async () => ({
+    crossOriginIsolated,
+    sharedArrayBuffer: typeof SharedArrayBuffer,
+    hardwareConcurrency: navigator.hardwareConcurrency,
+    deviceMemory: navigator.deviceMemory ?? null,
+    resources: performance.getEntriesByType('resource')
+      .filter((entry) => entry.name.includes('/vendor/ffmpeg'))
+      .map((entry) => ({
+        name: entry.name,
+        duration: entry.duration,
+        transferSize: entry.transferSize,
+        decodedBodySize: entry.decodedBodySize
+      })),
+    vendorFetches: await Promise.all([
+      '/vendor/ffmpeg/ffmpeg-core.js',
+      '/vendor/ffmpeg/ffmpeg-core.wasm',
+      '/vendor/ffmpeg/ffmpeg-core.worker.js',
+      '/vendor/ffmpeg-class/worker.js'
+    ].map(async (url) => {
+      try {
+        const response = await fetch(url);
+        return { url, ok: response.ok, status: response.status, length: Number(response.headers.get('content-length')) || null };
+      } catch (error) {
+        return { url, error: String(error) };
+      }
+    }))
+  }))()`);
+  console.log(JSON.stringify(diagnostics, null, 2));
+} else if (command === "bounds") {
+  console.log(
+    JSON.stringify(
+      await client.evaluate(`({
+        x: window.screenX,
+        y: window.screenY,
+        width: window.outerWidth,
+        height: window.outerHeight,
+        innerWidth: window.innerWidth,
+        innerHeight: window.innerHeight
+      })`),
+      null,
+      2
+    )
+  );
+} else if (command === "set-bounds") {
+  const match = /^(\d+)x(\d+)$/.exec(argument ?? "");
+  if (!match) throw new Error("set-bounds expects WIDTHxHEIGHT");
+  await client.evaluate(
+    `window.resizeTo(${Number(match[1])}, ${Number(match[2])}); true`
+  );
+  await sleep(500);
+  console.log(
+    JSON.stringify(
+      await client.evaluate(`({
+        x: window.screenX,
+        y: window.screenY,
+        width: window.outerWidth,
+        height: window.outerHeight,
+        innerWidth: window.innerWidth,
+        innerHeight: window.innerHeight
+      })`),
+      null,
+      2
+    )
+  );
+} else if (command === "close-window") {
+  console.log(JSON.stringify(await client.evaluate(`window.close(); true`)));
+} else if (command === "screenshot") {
+  const outputPath = argument ?? "rescript-e2e.png";
+  await client.send("Page.enable");
+  const capture = await client.send("Page.captureScreenshot", { format: "png", captureBeyondViewport: false });
+  await writeFile(outputPath, Buffer.from(capture.data, "base64"));
+  console.log(outputPath);
+} else {
+  throw new Error(`Unknown command: ${command}`);
+}
+
+client.close();
diff --git a/tests/i18n-test.ts b/tests/i18n-test.ts
new file mode 100644
index 0000000..2de06b8
--- /dev/null
+++ b/tests/i18n-test.ts
@@ -0,0 +1,48 @@
+import {
+  formatRelativeTime,
+  localizeRuntimeMessage,
+  resolveUiLocale,
+  translate,
+} from "../lib/i18n";
+
+function assert(value: unknown, message: string): asserts value {
+  if (!value) throw new Error(message);
+}
+
+assert(resolveUiLocale("system", ["zh-CN"]) === "zh-CN", "zh-CN detection");
+assert(resolveUiLocale("system", ["zh-HK"]) === "zh-CN", "zh-HK fallback");
+assert(resolveUiLocale("system", ["fr-FR", "en-US"]) === "en", "ordered fallback");
+assert(resolveUiLocale("system", ["fr-FR", "zh-Hans"]) === "zh-CN", "secondary zh");
+assert(resolveUiLocale("system", []) === "en", "empty fallback");
+assert(resolveUiLocale("zh-CN", ["en-US"]) === "zh-CN", "manual zh override");
+assert(resolveUiLocale("en", ["zh-CN"]) === "en", "manual en override");
+
+assert(translate("zh-CN", "common.settings") === "设置", "Chinese settings");
+assert(translate("en", "common.settings") === "Settings", "English settings");
+assert(
+  translate("zh-CN", "export.downloadFile", { name: "demo.mp4" }) ===
+    "下载 demo.mp4",
+  "named interpolation"
+);
+
+const zh = (key: Parameters<typeof translate>[1], params?: Record<string, string | number>) =>
+  translate("zh-CN", key, params);
+assert(
+  localizeRuntimeMessage("Transcribing…", zh) === "正在转录…",
+  "runtime progress localization"
+);
+assert(
+  localizeRuntimeMessage("Detecting language…", zh) === "正在识别源语言…",
+  "language detection progress localization"
+);
+assert(
+  localizeRuntimeMessage("No words to export.", zh) === "没有可导出的文字。",
+  "runtime error localization"
+);
+assert(localizeRuntimeMessage("Unknown diagnostic", zh) === "Unknown diagnostic", "fallback");
+
+const now = Date.UTC(2026, 7, 9, 12, 0, 0);
+assert(formatRelativeTime("zh-CN", now - 5 * 60_000, now).includes("5"), "zh relative");
+assert(formatRelativeTime("en", now - 5 * 60_000, now).includes("5"), "en relative");
+
+console.log("ALL I18N TESTS PASSED");
diff --git a/tests/languages-test.ts b/tests/languages-test.ts
index b10e6b9..d3a9151 100644
--- a/tests/languages-test.ts
+++ b/tests/languages-test.ts
@@ -1,15 +1,20 @@
 import {
   DEFAULT_TRANSCRIPT_LANGUAGE,
   isTranscriptLanguage,
+  isTranscriptLanguagePreference,
+  TRANSCRIPT_LANGUAGE_PREFERENCE_ORDER,
   TRANSCRIPT_LANGUAGE_ORDER,
   TRANSCRIPT_LANGUAGES,
 } from "../lib/languages";
 
 {
-  if (DEFAULT_TRANSCRIPT_LANGUAGE !== "en") {
-    throw new Error("expected default language to be en");
+  if (DEFAULT_TRANSCRIPT_LANGUAGE !== "auto") {
+    throw new Error("expected default language preference to be auto");
   }
   if (isTranscriptLanguage("auto")) throw new Error("did not expect auto to be valid");
+  if (!isTranscriptLanguagePreference("auto")) {
+    throw new Error("expected auto to be a valid preference");
+  }
   if (!isTranscriptLanguage("en")) throw new Error("expected en to be valid");
   if (!isTranscriptLanguage("es")) throw new Error("expected es to be valid");
   if (!isTranscriptLanguage("fr")) throw new Error("expected fr to be valid");
@@ -18,6 +23,14 @@ import {
   if (isTranscriptLanguage("pt")) throw new Error("did not expect pt to be valid");
 }
 
+{
+  if (TRANSCRIPT_LANGUAGE_PREFERENCE_ORDER.join(",") !== "auto,en,es,fr,de,zh") {
+    throw new Error(
+      `unexpected preference order: ${TRANSCRIPT_LANGUAGE_PREFERENCE_ORDER.join(",")}`
+    );
+  }
+}
+
 {
   const labels = TRANSCRIPT_LANGUAGE_ORDER.map(
     (id) => TRANSCRIPT_LANGUAGES[id].nativeLabel
diff --git a/tests/transcription-options-test.ts b/tests/transcription-options-test.ts
new file mode 100644
index 0000000..4d1f0b5
--- /dev/null
+++ b/tests/transcription-options-test.ts
@@ -0,0 +1,29 @@
+import {
+  resolveTranscriptLanguage,
+  whisperLanguageOptions,
+} from "../lib/transcriptionOptions";
+import type { TranscriptLanguage } from "../lib/languages";
+
+function assert(value: unknown, message: string): asserts value {
+  if (!value) throw new Error(message);
+}
+
+assert(resolveTranscriptLanguage("auto") === undefined, "auto must omit language");
+assert(resolveTranscriptLanguage("zh") === "zh", "zh must remain concrete");
+
+const automatic = whisperLanguageOptions(undefined);
+assert(automatic.task === "transcribe", "auto must transcribe");
+assert(!("language" in automatic), "auto must not contain a language key");
+
+for (const language of ["en", "es", "fr", "de", "zh"] satisfies TranscriptLanguage[]) {
+  const options = whisperLanguageOptions(language);
+  assert(options.task === "transcribe", `${language} must transcribe`);
+  assert(options.language === language, `${language} must be passed through`);
+  assert(options.task !== ("translate" as "transcribe"), "translate must never be used");
+}
+
+const detected = whisperLanguageOptions("ja");
+assert(detected.task === "transcribe", "a detected language must still transcribe");
+assert(detected.language === "ja", "a detected Whisper language code must pass through");
+
+console.log("ALL TRANSCRIPTION OPTIONS TESTS PASSED");
diff --git a/types/rescript-desktop.d.ts b/types/rescript-desktop.d.ts
index afeaf4f..bc657be 100644
--- a/types/rescript-desktop.d.ts
+++ b/types/rescript-desktop.d.ts
@@ -1,4 +1,4 @@
-/** Resting sizes the Electron shell switches between. */
+/** Renderer modes used by the Electron shell's two-step native Close behavior. */
 export type WindowMode = "compact" | "expanded";
 
 /** Actions the native File menu delegates to the renderer over IPC. Opening the
@@ -18,10 +18,12 @@ export interface RescriptDesktop {
     chrome: string;
     node: string;
   };
-  /** Resize the shell: "compact" for the upload screen, "expanded" for the editor. */
+  /** Report whether the upload screen or editor is active; this does not resize the window. */
   setWindowMode: (mode: WindowMode) => void;
   /** Mirror the telemetry opt-out to the main process, which gates its own reporting. */
   setTelemetryEnabled: (enabled: boolean) => void;
+  /** Synchronize the renderer's effective language with native menus/dialogs. */
+  setUiLocale: (locale: "en" | "zh-CN") => void;
   /** Publish the saved-project list (newest first) for File › Recent Projects. */
   setRecentProjects: (projects: Array<{ id: string; name: string }>) => void;
   /** Subscribe to File-menu actions; returns an unsubscribe function. */
diff --git a/workers/transcription.worker.ts b/workers/transcription.worker.ts
index 8da612f..237680c 100644
--- a/workers/transcription.worker.ts
+++ b/workers/transcription.worker.ts
@@ -23,6 +23,8 @@ import {
   AutoModelForCTC,
   AutoTokenizer,
   WhisperTextStreamer,
+  LogitsProcessor,
+  LogitsProcessorList,
   Tensor,
   env,
   type AutomaticSpeechRecognitionPipeline,
@@ -72,6 +74,7 @@ import {
   type CtcVocab,
 } from "@/lib/forcedAlign";
 import type { TranscriptLanguage } from "@/lib/languages";
+import { whisperLanguageOptions } from "@/lib/transcriptionOptions";
 import {
   VAD_FRAME_SIZE,
   VAD_SAMPLE_RATE,
@@ -149,6 +152,75 @@ const WHISPER_LEAD_PAD_S = 0.5;
 
 type AsrChunk = { text: string; timestamp: [number, number | null] };
 
+/** Restrict Whisper's first generated token to its language-token vocabulary. */
+class WhisperLanguageTokenProcessor extends LogitsProcessor {
+  private readonly allowed: Set<number>;
+
+  constructor(tokenIds: number[]) {
+    super();
+    this.allowed = new Set(tokenIds);
+  }
+
+  _call(_inputIds: bigint[][], logits: Tensor): Tensor {
+    const values = logits.data as Float32Array;
+    const vocabularySize = logits.dims.at(-1) ?? 0;
+    for (let index = 0; index < values.length; index++) {
+      if (!this.allowed.has(index % vocabularySize)) values[index] = -Infinity;
+    }
+    return logits;
+  }
+}
+
+/**
+ * Transformers.js 4.2 does not yet implement Whisper language detection: when
+ * `language` is omitted it explicitly defaults to English. Run Whisper's
+ * standard first-token probe instead, constrained to every language token in
+ * the checkpoint, then feed the detected code back into source-language
+ * transcription.
+ */
+async function detectWhisperLanguage(
+  transcriber: AutomaticSpeechRecognitionPipeline,
+  audio: Float32Array
+): Promise<string> {
+  const generationConfig = transcriber.model.generation_config as {
+    decoder_start_token_id?: number | null;
+    lang_to_id?: Record<string, number> | null;
+  };
+  const startToken = generationConfig.decoder_start_token_id;
+  const langToId = generationConfig.lang_to_id;
+  if (startToken == null || !langToId) {
+    throw new Error("This Whisper model does not expose language detection tokens.");
+  }
+
+  const tokenToLanguage = new Map<number, string>();
+  for (const [token, id] of Object.entries(langToId)) {
+    const match = /^<\|([a-z]{2,3})\|>$/.exec(token);
+    if (match) tokenToLanguage.set(id, match[1]);
+  }
+  if (tokenToLanguage.size === 0) {
+    throw new Error("This Whisper model has no usable language tokens.");
+  }
+
+  const { input_features } = await transcriber.processor(audio);
+  const languageProcessors = new LogitsProcessorList();
+  languageProcessors.push(new WhisperLanguageTokenProcessor([...tokenToLanguage.keys()]));
+  const generated = (await transcriber.model.generate({
+    inputs: input_features,
+    decoder_input_ids: new Tensor(
+      "int64",
+      BigInt64Array.of(BigInt(startToken)),
+      [1, 1]
+    ),
+    max_new_tokens: 1,
+    do_sample: false,
+    logits_processor: languageProcessors,
+  })) as Tensor;
+  const sequence = generated.tolist()[0] as bigint[];
+  const language = tokenToLanguage.get(Number(sequence.at(-1)));
+  if (!language) throw new Error("Whisper could not detect the source language.");
+  return language;
+}
+
 const post = (msg: WorkerResponse, transfer: Transferable[] = []) =>
   (self as unknown as Worker).postMessage(msg, transfer);
 
@@ -953,7 +1025,7 @@ async function refineWordTimestamps(
   speechFrames: boolean[],
   audio: Float32Array,
   duration: number,
-  language: TranscriptLanguage
+  language: TranscriptLanguage | undefined
 ): Promise<Word[]> {
   let out = alignWordsToSpeech(words, speechFrames, {
     duration,
@@ -961,7 +1033,7 @@ async function refineWordTimestamps(
     sampleRate: VAD_SAMPLE_RATE,
   });
 
-  if (alignModelFor(language)) {
+  if (language && alignModelFor(language)) {
     try {
       const measured = await forceAlign(out, audio, duration, language);
       out = expandToAcoustics(measured, speechEnvelope(audio, VAD_SAMPLE_RATE));
@@ -1122,11 +1194,11 @@ async function finishWithDiarization(
 async function runParakeet(
   audio: Float32Array,
   duration: number,
-  transcriptLanguage: TranscriptLanguage
+  transcriptLanguage: TranscriptLanguage | undefined
 ): Promise<Word[]> {
   // Overlap diarizer (+ language-matched aligner) with Parakeet load.
   getDiarizer().catch(() => {});
-  if (alignModelFor(transcriptLanguage)) {
+  if (transcriptLanguage && alignModelFor(transcriptLanguage)) {
     getAligner(transcriptLanguage).catch(() => {});
   }
   const [loaded, vad] = await Promise.all([getParakeet(), getVad()]);
@@ -1208,12 +1280,12 @@ async function runWhisper(
   audio: Float32Array,
   duration: number,
   choice: WhisperModel,
-  transcriptLanguage: TranscriptLanguage
+  transcriptLanguage: TranscriptLanguage | undefined
 ): Promise<Word[]> {
   // Overlap Whisper + Silero downloads; diarizer and language-matched aligner
   // warm in the background so both are cached by the time the transcript lands.
   getDiarizer().catch(() => {});
-  if (alignModelFor(transcriptLanguage)) {
+  if (transcriptLanguage && alignModelFor(transcriptLanguage)) {
     getAligner(transcriptLanguage).catch(() => {});
   }
   const [asr, vad] = await Promise.all([getAsr(choice), getVad()]);
@@ -1228,6 +1300,21 @@ async function runWhisper(
     0
   );
 
+  let whisperLanguage: string | undefined = transcriptLanguage;
+  if (!whisperLanguage && speechSegments.length > 0) {
+    post({ type: "progress", message: "Detecting language…", value: null });
+    const probe = speechSegments.reduce((longest, segment) =>
+      segment.endSample - segment.startSample > longest.endSample - longest.startSample
+        ? segment
+        : longest
+    );
+    const probeEnd = Math.min(probe.endSample, probe.startSample + 30 * VAD_SAMPLE_RATE);
+    whisperLanguage = await detectWhisperLanguage(
+      transcriber,
+      audio.slice(probe.startSample, probeEnd)
+    );
+  }
+
   post({ type: "progress", message: "Transcribing…", value: 0 });
 
   let partial = "";
@@ -1294,7 +1381,7 @@ async function runWhisper(
     // short VAD segments on every model here, whether with Whisper's
     // <|startofprev|> filler prompt or CrisperWhisper's mode tags. See the note
     // above MODELS in lib/models.ts; vad-regression-test.ts guards it.
-    language: transcriptLanguage,
+    ...whisperLanguageOptions(whisperLanguage),
   };
 
   const rawWords: Word[] = [];
@@ -1409,7 +1496,7 @@ self.onmessage = async (event: MessageEvent<WorkerRequest>) => {
   const { audio, duration, model, language } = event.data;
   try {
     const choice: ModelId = model ?? "base";
-    const transcriptLanguage: TranscriptLanguage = language ?? "en";
+    const transcriptLanguage: TranscriptLanguage | undefined = language;
 
     let words: Word[];
     if (isParakeetModel(choice)) {