gh-christianlouis-docuelevate/docs/InternationalizationGuide.md

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

# 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:

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:

{
  "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:

{# 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

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:

{# In templates — locale is automatically detected #}
<span>{{ format_date_l10n(document.created_at) }}</span>
<span>{{ format_number_l10n(file_count) }}</span>

# 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:

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:

   {"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:

{
  "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:

{"language": "de"}

Response:

{
  "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