test(i18n): add comprehensive tests and documentation for i18n system

- Add 45 tests covering translation files, translate(), Accept-Language parsing, language detection, l10n formatters, and API endpoints - Create InternationalizationGuide.md documentation - Fix linting issues (E741, PLW2901) Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
2026-03-09 23:35:32 +00:00
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+# Internationalization (i18n) & Localization (l10n) Guide
+
+DocuElevate supports **10 languages** for its web UI, with automatic browser
+language detection, user-preference persistence, and an AI-powered fallback
+translator for strings that haven't been manually translated yet.
+
+## Supported Languages
+
+| Code | Language    | Native Name | Priority |
+|------|------------|-------------|----------|
+| `en` | English    | English     | Tier 1   |
+| `de` | German     | Deutsch     | Tier 1   |
+| `fr` | French     | Français    | Tier 1   |
+| `es` | Spanish    | Español     | Tier 1   |
+| `it` | Italian    | Italiano    | Tier 1   |
+| `pt` | Portuguese | Português   | Tier 1   |
+| `nl` | Dutch      | Nederlands  | Tier 2   |
+| `pl` | Polish     | Polski      | Tier 2   |
+| `zh` | Chinese    | 中文         | Tier 2   |
+| `ru` | Russian    | Русский     | Tier 2   |
+
+> **Tier 1** languages (European priority) have complete, manually-reviewed
+> translations. **Tier 2** languages have complete translations but may
+> receive less frequent updates.
+
+## How Language Is Detected
+
+DocuElevate resolves the display language in the following priority order:
+
+1. **User profile preference** — stored in the database (`UserProfile.preferred_language`)
+   and loaded into the session on login
+2. **Cookie** — `docuelevate_lang` cookie (30-day expiry, set when user selects a language)
+3. **Browser `Accept-Language` header** — the highest-priority match among supported languages
+4. **Default** — English (`en`)
+
+## Selecting Your Language
+
+### Via the Navigation Bar
+
+Click the 🌐 **globe icon** in the top navigation bar. A dropdown menu shows all
+available languages with their native names and flag emoji. The current language
+is highlighted with a blue checkmark.
+
+### Via the API
+
+```bash
+# Set language to German
+curl -X POST http://localhost:8000/api/i18n/language \
+  -H "Content-Type: application/json" \
+  -d '{"language": "de"}'
+
+# List all available languages
+curl http://localhost:8000/api/i18n/languages
+```
+
+### Via Cookie (Programmatic)
+
+Set the `docuelevate_lang` cookie to any supported language code:
+
+```javascript
+document.cookie = "docuelevate_lang=fr; max-age=2592000; path=/";
+location.reload();
+```
+
+## For Developers
+
+### Translation File Structure
+
+Translations are stored as flat JSON files in `frontend/translations/`:
+
+```
+frontend/translations/
+├── en.json    # English (base / reference)
+├── de.json    # German
+├── fr.json    # French
+├── es.json    # Spanish
+├── it.json    # Italian
+├── pt.json    # Portuguese
+├── nl.json    # Dutch
+├── pl.json    # Polish
+├── zh.json    # Chinese
+└── ru.json    # Russian
+```
+
+Each file is a flat key-value dictionary with dot-notation namespacing:
+
+```json
+{
+  "nav.dashboard": "Dashboard",
+  "nav.upload": "Upload",
+  "upload.max_size": "Maximum file size: {size}",
+  "footer.copyright": "DocuElevate {year}"
+}
+```
+
+### Using Translations in Templates
+
+The `_()` function is available globally in all Jinja2 templates:
+
+```jinja2
+{# Simple translation #}
+<h1>{{ _("dashboard.title") }}</h1>
+
+{# Translation with placeholders #}
+<p>{{ _("upload.max_size", size="50 MB") }}</p>
+
+{# Translation in attributes #}
+<button aria-label="{{ _('common.save') }}">{{ _("common.save") }}</button>
+```
+
+### Using Translations in Python
+
+```python
+from app.utils.i18n import translate
+
+# Basic translation
+text = translate("nav.dashboard", "de")  # → "Übersicht"
+
+# With placeholders
+text = translate("footer.copyright", "fr", year="2025")  # → "DocuElevate 2025"
+```
+
+### Localization Helpers
+
+Format dates, times, and numbers according to locale conventions:
+
+```jinja2
+{# In templates — locale is automatically detected #}
+<span>{{ format_date_l10n(document.created_at) }}</span>
+<span>{{ format_number_l10n(file_count) }}</span>
+```
+
+```python
+# In Python
+from app.utils.i18n import format_date, format_number
+
+format_date(date(2025, 3, 15), "de")      # → "15. March 2025"
+format_date(date(2025, 3, 15), "de", short=True)  # → "15.03.2025"
+format_number(1234567, "de")                # → "1.234.567"
+format_number(1234.56, "en")                # → "1,234.56"
+```
+
+### Adding a New Translation Key
+
+1. Add the key and English text to `frontend/translations/en.json`
+2. Add translations for all other languages in their respective files
+3. Use `{{ _("your.new.key") }}` in templates
+
+### AI Fallback Translation
+
+When a translation key exists in English but not in the target language,
+DocuElevate can use the configured AI provider (OpenAI, Anthropic, etc.)
+to translate the string on-the-fly:
+
+```python
+from app.utils.i18n import translate_with_ai_fallback
+
+# Falls back to AI if no manual translation exists
+translated = translate_with_ai_fallback("Welcome to our platform", "de")
+```
+
+The AI fallback:
+- Uses the `AI_MODEL` or `OPENAI_MODEL` setting
+- Caches results in memory for the process lifetime
+- Returns the original English text if the AI call fails
+- Is designed for graceful degradation — the UI never breaks
+
+### Adding a New Language
+
+1. Create a new JSON file in `frontend/translations/` (e.g., `ja.json`)
+2. Copy the structure from `en.json` and translate all values
+3. Add the language to `SUPPORTED_LANGUAGES` in `app/utils/i18n.py`:
+   ```python
+   {"code": "ja", "name": "Japanese", "native": "日本語", "flag": "🇯🇵"},
+   ```
+4. Add locale formatting rules to `_LOCALE_FORMATS` in the same file
+5. Create a database migration if needed (the `preferred_language` column
+   already accepts any string up to 10 characters)
+
+### Database Migration
+
+Migration `027_add_user_language_preference` adds a `preferred_language`
+column to the `user_profiles` table. This column stores the user's chosen
+UI language as an ISO 639-1 code (e.g., `"de"`, `"fr"`). A `NULL` value
+means "auto-detect from browser settings."
+
+### API Reference
+
+#### `GET /api/i18n/languages`
+
+Returns all supported languages and the current active language.
+
+**Response:**
+```json
+{
+  "languages": [
+    {"code": "en", "name": "English", "native": "English", "flag": "🇬🇧"},
+    {"code": "de", "name": "German", "native": "Deutsch", "flag": "🇩🇪"}
+  ],
+  "current": "en",
+  "default": "en"
+}
+```
+
+#### `POST /api/i18n/language`
+
+Set the preferred UI language. Persists in session, cookie, and database.
+
+**Request:**
+```json
+{"language": "de"}
+```
+
+**Response:**
+```json
+{
+  "language": "de",
+  "message": "Language changed to Deutsch"
+}
+```
+
+## Configuration
+
+No additional configuration is required. The i18n system works out of the box
+with the default English language and automatically detects browser preferences.
+
+| Setting | Default | Description |
+|---------|---------|-------------|
+| Browser `Accept-Language` | Auto-detected | Used when no explicit preference is set |
+| `docuelevate_lang` cookie | Not set | Set when user selects a language via the UI |
+| `UserProfile.preferred_language` | `NULL` | Stored in DB for authenticated users |