diff --git a/.dockerignore b/.dockerignore
index 08e0db2..b512aa6 100644
--- a/.dockerignore
+++ b/.dockerignore
@@ -23,13 +23,30 @@ frontend/out
 frontend/.env*
 frontend/*.log
 
-# Project
+# Project data
 .antigravity
 .gemini
 tmp
 data
 mydata
+notebook_data
+surreal_data
+surreal-data
+surreal_single_data
 *.db
 *.log
 docker.env
-.env
\ No newline at end of file
+.env
+docker-compose*
+
+# Documentation & CI (not needed in image)
+docs
+.github
+
+# IDE and OS files
+.vscode
+.idea
+*.swp
+*.swo
+*~
+.DS_Store
\ No newline at end of file
diff --git a/CHANGELOG.md b/CHANGELOG.md
index ebd293f..d6b2ecc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,25 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [1.5.0] - 2026-01-15
+
+### Added
+- Internationalization (i18n) support with Chinese (Simplified and Traditional) translations (#371, closes #344, #349, #360)
+- Frontend test infrastructure with Vitest (#371)
+- Language toggle component for switching UI language (#371)
+- Date localization using date-fns locales (#371)
+- Error message translation system (#371)
+
+### Fixed
+- Accessibility improvements: added missing `id`, `name`, and `autoComplete` attributes to form inputs (#371)
+- Added `DialogDescription` to dialogs for Radix UI accessibility compliance (#371)
+- Fixed "Collapsible is changing from uncontrolled to controlled" warning in SettingsForm (#371)
+- Fixed lint command for Next.js 16 compatibility (`eslint` instead of `next lint`)
+
+### Changed
+- Dockerfile optimizations: better layer caching, `--no-install-recommends` for smaller images (#371)
+- Dockerfile.single refactored into 3 separate build stages for better caching (#371)
+
 ## [1.4.0] - 2026-01-14
 
 ### Added
diff --git a/README.dev.md b/README.dev.md
index 1c7d27e..eac28b0 100644
--- a/README.dev.md
+++ b/README.dev.md
@@ -244,6 +244,47 @@ uv sync
 cd frontend && npm install package-name
 ```
 
+### Adding a New Language (i18n)
+
+Open Notebook supports internationalization. To add a new language:
+
+1. **Create locale file**: Copy an existing locale as template
+   ```bash
+   cp frontend/src/lib/locales/en-US/index.ts frontend/src/lib/locales/pt-BR/index.ts
+   ```
+
+2. **Translate all strings** in the new file. The structure includes:
+   - `common`: Shared UI elements (buttons, labels)
+   - `notebooks`, `sources`, `notes`: Feature-specific strings
+   - `chat`, `search`, `podcasts`: Module-specific strings
+   - `apiErrors`: Error message translations
+
+3. **Register the locale** in `frontend/src/lib/locales/index.ts`:
+   ```typescript
+   import { ptBR } from './pt-BR'
+
+   export const locales = {
+     'en-US': enUS,
+     'zh-CN': zhCN,
+     'zh-TW': zhTW,
+     'pt-BR': ptBR,  // Add your locale
+   }
+   ```
+
+4. **Add date-fns locale** in `frontend/src/lib/utils/date-locale.ts`:
+   ```typescript
+   import { zhCN, enUS, zhTW, ptBR } from 'date-fns/locale'
+
+   const LOCALE_MAP: Record<string, Locale> = {
+     'zh-CN': zhCN,
+     'zh-TW': zhTW,
+     'en-US': enUS,
+     'pt-BR': ptBR,  // Add your locale
+   }
+   ```
+
+5. **Test**: Switch languages using the language toggle in the UI header.
+
 ### Database Migrations
 
 Database migrations run **automatically** when the API starts.
diff --git a/README.md b/README.md
index 05a1660..e4f72ec 100644
--- a/README.md
+++ b/README.md
@@ -63,6 +63,7 @@ In a world dominated by Artificial Intelligence, having the ability to think 
 - 🎙️ **Generate professional podcasts** - Advanced multi-speaker podcast generation
 - 🔍 **Search intelligently** - Full-text and vector search across all your content
 - 💬 **Chat with context** - AI conversations powered by your research
+- 🌐 **Multi-language UI** - Chinese (Simplified & Traditional) support, more languages coming soon!
 
 Learn more about our project at [https://www.open-notebook.ai](https://www.open-notebook.ai)
 
diff --git a/frontend/package.json b/frontend/package.json
index 3e0e6e9..f63dd24 100644
--- a/frontend/package.json
+++ b/frontend/package.json
@@ -6,7 +6,7 @@
     "dev": "next dev",
     "build": "next build",
     "start": "node start-server.js",
-    "lint": "next lint",
+    "lint": "eslint src/",
     "test": "vitest run",
     "test:watch": "vitest",
     "test:ui": "vitest --ui"
diff --git a/frontend/src/CLAUDE.md b/frontend/src/CLAUDE.md
index c02e0fd..3dd7cbc 100644
--- a/frontend/src/CLAUDE.md
+++ b/frontend/src/CLAUDE.md
@@ -15,6 +15,7 @@ User interactions trigger mutations/queries via hooks, which communicate with th
 - **`lib/api/CLAUDE.md`**: Axios client, FormData handling, interceptors
 - **`lib/hooks/CLAUDE.md`**: TanStack Query wrappers, SSE streaming, context building
 - **`lib/stores/CLAUDE.md`**: Zustand auth/modal state, localStorage persistence
+- **`lib/locales/CLAUDE.md`**: Internationalization (i18n) system, translation files
 - **`components/ui/CLAUDE.md`**: Radix UI primitives, CVA styling, accessibility
 
 ## Architectural Layers
@@ -60,6 +61,12 @@ User interactions trigger mutations/queries via hooks, which communicate with th
 - API request/response shapes, domain models (Notebook, Source, Note, etc.)
 - Ensures type safety across API calls and store mutations
 
+#### `lib/locales/` — Internationalization (i18n)
+- **Locale files** (`en-US/`, `zh-CN/`, `zh-TW/`): Translation strings organized by feature
+- **`i18n.ts`**: i18next configuration with language detection
+- **`use-translation.ts`**: Custom hook with Proxy-based `t.section.key` access pattern
+- **Pattern**: Components call `useTranslation()` hook; access strings via `t.common.save`, `t.notebooks.title`
+
 ## Data & Control Flow Walkthrough
 
 ### Example: Notebook Chat
@@ -123,12 +130,13 @@ User interactions trigger mutations/queries via hooks, which communicate with th
 
 ## Providers & Context Setup
 
-**Root layout** (`app/layout.tsx`) wraps app with:
-1. `ThemeProvider` — next-themes for light/dark mode
-2. `QueryProvider` — TanStack Query client
-3. `ErrorBoundary` — React error boundary
-4. `ConnectionGuard` — checks backend connectivity on startup
-5. `Toaster` — sonner toast notification system
+**Root layout** (`app/layout.tsx`) wraps app with (outermost → innermost):
+1. `ErrorBoundary` — React error boundary (catches all render errors)
+2. `ThemeProvider` — next-themes for light/dark mode
+3. `QueryProvider` — TanStack Query client
+4. `I18nProvider` — i18next initialization and language loading overlay
+5. `ConnectionGuard` — checks backend connectivity on startup
+6. `Toaster` — sonner toast notification system (inside ConnectionGuard)
 
 ## Important Gotchas & Design Decisions
 
diff --git a/frontend/src/components/ui/alert-dialog.tsx b/frontend/src/components/ui/alert-dialog.tsx
index a9ba722..0863e40 100644
--- a/frontend/src/components/ui/alert-dialog.tsx
+++ b/frontend/src/components/ui/alert-dialog.tsx
@@ -53,7 +53,6 @@ function AlertDialogContent({
       <AlertDialogOverlay />
       <AlertDialogPrimitive.Content
         data-slot="alert-dialog-content"
-        aria-describedby={undefined}
         className={cn(
           "bg-background data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0 data-[state=closed]:zoom-out-95 data-[state=open]:zoom-in-95 fixed top-[50%] left-[50%] z-50 grid w-full max-w-[calc(100%-2rem)] translate-x-[-50%] translate-y-[-50%] gap-4 rounded-lg border p-6 shadow-lg duration-200 sm:max-w-lg",
           className
diff --git a/frontend/src/lib/hooks/CLAUDE.md b/frontend/src/lib/hooks/CLAUDE.md
index 2e969a5..a90a8b5 100644
--- a/frontend/src/lib/hooks/CLAUDE.md
+++ b/frontend/src/lib/hooks/CLAUDE.md
@@ -10,6 +10,7 @@ React hooks for API data fetching, state management, and complex workflows (chat
 - **Streaming hooks** (`useAsk`): SSE parsing for multi-stage Ask workflows (strategy → answers → final answer)
 - **Model/config hooks** (`useModels`, `useSettings`, `useTransformations`): Application-level settings and model management
 - **Utility hooks** (`useMediaQuery`, `useToast`, `useNavigation`, `useAuth`): UI state and auth checking
+- **i18n hook** (`useTranslation`): Proxy-based translation access with `t.section.key` pattern and language switching
 
 ## Important Patterns
 
@@ -21,6 +22,7 @@ React hooks for API data fetching, state management, and complex workflows (chat
 - **SSE streaming pattern**: `useAsk` manually parses newline-delimited JSON from `/api/search/ask`; handles incomplete buffers
 - **Status polling**: `useSourceStatus` auto-refetches every 2s while `status === 'running' | 'queued' | 'new'`
 - **Context building**: `useNotebookChat.buildContext()` assembles selected sources + notes with token/char counts
+- **i18n Proxy pattern**: `useTranslation` returns `t` object with Proxy; access `t.section.key` instead of `t('section.key')`
 
 ## Key Dependencies
 
@@ -47,6 +49,8 @@ React hooks for API data fetching, state management, and complex workflows (chat
 - **Status polling race**: `useSourceStatus` may refetch stale data before server catches up; retry logic has 3-attempt limit
 - **Keyboard trap in dialogs**: Some hooks manage modal state; ensure Dialog/Modal components handle escape key properly
 - **Form data handling**: `useFileUpload` and source creation convert JSON fields to strings in FormData
+- **useTranslation depth limit**: Proxy limits nesting to 4 levels; deeper access returns path string as fallback
+- **useTranslation loop detection**: >1000 accesses to same key in 1s triggers error and breaks recursion
 
 ## Testing Patterns
 
diff --git a/frontend/src/lib/locales/CLAUDE.md b/frontend/src/lib/locales/CLAUDE.md
new file mode 100644
index 0000000..7e430bd
--- /dev/null
+++ b/frontend/src/lib/locales/CLAUDE.md
@@ -0,0 +1,141 @@
+# Locales Module (i18n)
+
+Internationalization system providing multi-language UI support using i18next with type-safe translation access.
+
+## Architecture
+
+```
+lib/
+├── i18n.ts              # i18next initialization and configuration
+├── i18n-events.ts       # Language change event emitters
+├── hooks/
+│   └── use-translation.ts  # Custom hook with Proxy-based API
+├── utils/
+│   └── date-locale.ts   # date-fns locale mapping
+└── locales/
+    ├── index.ts         # Locale registry and type exports
+    ├── en-US/index.ts   # English translations
+    ├── zh-CN/index.ts   # Simplified Chinese translations
+    └── zh-TW/index.ts   # Traditional Chinese translations
+```
+
+## Key Components
+
+- **`i18n.ts`**: i18next initialization with language detection (localStorage → browser)
+- **`i18n-events.ts`**: Event emitters for language change start/end (used by loading overlay)
+- **`locales/index.ts`**: Central registry exporting all locales and `LanguageCode` type
+- **`use-translation.ts`**: Custom hook providing `t` object with nested property access
+
+## Translation Structure
+
+Each locale file exports a flat object with nested keys:
+
+```typescript
+export const enUS = {
+  common: {
+    save: 'Save',
+    cancel: 'Cancel',
+    delete: 'Delete',
+    // ...
+  },
+  notebooks: {
+    title: 'Notebooks',
+    createNew: 'Create Notebook',
+    // ...
+  },
+  // ... other sections
+}
+```
+
+**Sections**:
+- `common`: Shared UI elements (buttons, labels, actions)
+- `notebooks`, `sources`, `notes`: Feature-specific strings
+- `chat`, `search`, `podcasts`: Module-specific strings
+- `models`, `transformations`, `settings`: Configuration UI
+- `advanced`: System administration strings
+- `apiErrors`: Backend error message translations
+
+## Usage Pattern
+
+```typescript
+import { useTranslation } from '@/lib/hooks/use-translation'
+
+function MyComponent() {
+  const { t, language, setLanguage } = useTranslation()
+
+  // Nested property access (Proxy-based)
+  return <h1>{t.notebooks.title}</h1>
+
+  // With interpolation
+  return <p>{t.common.updated.replace('{time}', timeAgo)}</p>
+
+  // Change language
+  await setLanguage('zh-CN')
+}
+```
+
+## Important Patterns
+
+- **Proxy-based access**: `t.section.key` instead of `t('section.key')` for better DX
+- **Type safety**: `TranslationKeys` type derived from `enUS` locale
+- **Language persistence**: Saved to localStorage, auto-detected on load
+- **Fallback**: Falls back to `en-US` if key missing in current locale
+- **Date localization**: Use `getDateLocale(language)` from `utils/date-locale.ts`
+
+## Key Dependencies
+
+- `i18next`: Core internationalization framework
+- `react-i18next`: React bindings for i18next
+- `i18next-browser-languagedetector`: Auto-detect browser language
+- `date-fns/locale`: Date formatting locales
+
+## How to Add a New Language
+
+1. Create locale folder: `locales/pt-BR/index.ts`
+2. Copy structure from `en-US/index.ts` and translate all strings
+3. Register in `locales/index.ts`:
+   ```typescript
+   import { ptBR } from './pt-BR'
+   export const resources = {
+     // ...existing
+     'pt-BR': { translation: ptBR },
+   }
+   export const languages: Language[] = [
+     // ...existing
+     { code: 'pt-BR', label: 'Português' },
+   ]
+   ```
+4. Add to `utils/date-locale.ts`:
+   ```typescript
+   import { ptBR } from 'date-fns/locale'
+   const LOCALE_MAP = { ...existing, 'pt-BR': ptBR }
+   ```
+
+## Important Quirks & Gotchas
+
+- **Proxy depth limit**: `useTranslation` limits nesting to 4 levels to prevent infinite loops
+- **Blocked properties**: React internals (`__proto__`, `$$typeof`, etc.) are blocked from Proxy traversal
+- **Loop detection**: Access counts reset every 1s; >1000 accesses triggers error and breaks recursion
+- **String methods**: `.replace()`, `.split()` work on translated strings via Proxy magic
+- **Language change events**: `emitLanguageChangeStart/End` used by `LanguageLoadingOverlay` for UX
+- **No SSR**: `useSuspense: false` disables React Suspense for i18next (avoids hydration issues)
+- **All keys required**: Missing keys in non-English locales fall back to English; keep locales in sync
+
+## Testing Patterns
+
+```typescript
+// Mock useTranslation in tests (see test/setup.ts)
+vi.mock('@/lib/hooks/use-translation', () => ({
+  useTranslation: () => ({
+    t: enUS,  // Use English locale directly
+    language: 'en-US',
+    setLanguage: vi.fn(),
+  }),
+}))
+
+// Test locale completeness
+import { enUS, zhCN } from '@/lib/locales'
+const enKeys = Object.keys(flatten(enUS))
+const zhKeys = Object.keys(flatten(zhCN))
+expect(zhKeys).toEqual(enKeys)  // All keys present
+```
diff --git a/pyproject.toml b/pyproject.toml
index bf23385..42ebda6 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [project]
 name = "open-notebook"
-version = "1.4.0"
+version = "1.5.0"
 description = "An open source implementation of a research assistant, inspired by Google Notebook LM"
 authors = [
     {name = "Luis Novo", email = "lfnovo@gmail.com"}
diff --git a/uv.lock b/uv.lock
index 308032f..58f3c23 100644
--- a/uv.lock
+++ b/uv.lock
@@ -2375,7 +2375,7 @@ wheels = [
 
 [[package]]
 name = "open-notebook"
-version = "1.4.0"
+version = "1.5.0"
 source = { editable = "." }
 dependencies = [
     { name = "ai-prompter" },