📝(docs) Add language configuration documentation

Add comprehensive guide explaining how to override LANGUAGES settings
using the DJANGO_LANGUAGES environment variable. Documentation includes:

- Default language configuration
- Environment variable format and examples
- Configuration for development, production, and Docker Compose
- Complete list of 15 available languages with translation files
- Language code formatting guidelines
- Testing and troubleshooting sections

Author: AntoLC

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to
 
 - ✨(backend) add documents/all endpoint with descendants #1553
 - ✅(export) add PDF regression tests #1762
+- 📝(docs) Add language configuration documentation #1757
 
 ### Fixed 
 

docs/languages-configuration.md

@@ -0,0 +1,180 @@
+# Language Configuration (2025-12)
+
+This document explains how to configure and override the available languages in the Docs application.
+
+## Default Languages
+
+By default, the application supports the following languages (in priority order):
+
+- English (en-us)
+- French (fr-fr)
+- German (de-de)
+- Dutch (nl-nl)
+- Spanish (es-es)
+
+The default configuration is defined in `src/backend/impress/settings.py`:
+
+```python
+LANGUAGES = values.SingleNestedTupleValue(
+    (
+        ("en-us", "English"),
+        ("fr-fr", "Français"),
+        ("de-de", "Deutsch"),
+        ("nl-nl", "Nederlands"),
+        ("es-es", "Español"),
+    )
+)
+```
+
+## Overriding Languages
+
+### Using Environment Variables
+
+You can override the available languages by setting the `DJANGO_LANGUAGES` environment variable. This is the recommended approach for customizing language support without modifying the source code.
+
+#### Format
+
+The `DJANGO_LANGUAGES` variable expects a semicolon-separated list of language configurations, where each language is defined as `code,Display Name`:
+
+```
+DJANGO_LANGUAGES=code1,Name1;code2,Name2;code3,Name3
+```
+
+#### Example Configurations
+
+**Example 1: English and French only**
+
+```bash
+DJANGO_LANGUAGES=en-us,English;fr-fr,Français
+```
+
+**Example 2: Add Italian and Chinese**
+
+```bash
+DJANGO_LANGUAGES=en-us,English;fr-fr,Français;de-de,Deutsch;it-it,Italiano;zh-cn,中文
+```
+
+**Example 3: Custom subset of languages**
+
+```bash
+DJANGO_LANGUAGES=fr-fr,Français;de-de,Deutsch;es-es,Español
+```
+
+### Configuration Files
+
+#### Development Environment
+
+For local development, you can set the `DJANGO_LANGUAGES` variable in your environment configuration file:
+
+**File:** `env.d/development/common.local`
+
+```bash
+DJANGO_LANGUAGES=en-us,English;fr-fr,Français;de-de,Deutsch;it-it,Italiano;zh-cn,中文;
+```
+
+#### Production Environment
+
+For production deployments, add the variable to your production environment configuration:
+
+**File:** `env.d/production.dist/common`
+
+```bash
+DJANGO_LANGUAGES=en-us,English;fr-fr,Français
+```
+
+#### Docker Compose
+
+When using Docker Compose, you can set the environment variable in your `compose.yml` or `compose.override.yml` file:
+
+```yaml
+services:
+  app:
+    environment:
+      - DJANGO_LANGUAGES=en-us,English;fr-fr,Français;de-de,Deutsch
+```
+
+## Important Considerations
+
+### Language Codes
+
+- Use standard language codes (ISO 639-1 with optional region codes)
+- Format: `language-region` (e.g., `en-us`, `fr-fr`, `de-de`)
+- Use lowercase for language codes and region identifiers
+
+### Priority Order
+
+Languages are listed in priority order. The first language in the list is used as the fallback language throughout the application when a specific translation is not available.
+
+### Translation Availability
+
+Before adding a new language, ensure that:
+
+1. Translation files exist for that language in the `src/backend/locale/` directory
+2. The frontend application has corresponding translation files
+3. All required messages have been translated
+
+#### Available Languages
+
+The following languages have translation files available in `src/backend/locale/`:
+
+- `br_FR` - Breton (France)
+- `cn_CN` - Chinese (China) - *Note: Use `zh-cn` in DJANGO_LANGUAGES*
+- `de_DE` - German (Germany) - Use `de-de`
+- `en_US` - English (United States) - Use `en-us`
+- `es_ES` - Spanish (Spain) - Use `es-es`
+- `fr_FR` - French (France) - Use `fr-fr`
+- `it_IT` - Italian (Italy) - Use `it-it`
+- `nl_NL` - Dutch (Netherlands) - Use `nl-nl`
+- `pt_PT` - Portuguese (Portugal) - Use `pt-pt`
+- `ru_RU` - Russian (Russia) - Use `ru-ru`
+- `sl_SI` - Slovenian (Slovenia) - Use `sl-si`
+- `sv_SE` - Swedish (Sweden) - Use `sv-se`
+- `tr_TR` - Turkish (Turkey) - Use `tr-tr`
+- `uk_UA` - Ukrainian (Ukraine) - Use `uk-ua`
+- `zh_CN` - Chinese (China) - Use `zh-cn`
+
+**Note:** When configuring `DJANGO_LANGUAGES`, use lowercase with hyphens (e.g., `pt-pt`, `ru-ru`) rather than the directory name format.
+
+### Translation Management
+
+We use [Crowdin](https://crowdin.com/) to manage translations for the Docs application. Crowdin allows our community to contribute translations and helps maintain consistency across all supported languages.
+
+**Want to add a new language or improve existing translations?**
+
+If you would like us to support a new language or want to contribute to translations, please get in touch with the project maintainers. We can add new languages to our Crowdin project and coordinate translation efforts with the community.
+
+### Cookie and Session
+
+The application stores the user's language preference in a cookie named `docs_language`. The cookie path is set to `/` by default.
+
+## Testing Language Configuration
+
+After changing the language configuration:
+
+1. Restart the application services
+2. Verify the language selector displays the correct languages
+3. Test switching between different languages
+4. Confirm that content is displayed in the selected language
+
+## Troubleshooting
+
+### Languages not appearing
+
+- Verify the environment variable is correctly formatted (semicolon-separated, comma between code and name)
+- Check that there are no trailing spaces in language codes or names
+- Ensure the application was restarted after changing the configuration
+
+### Missing translations
+
+If you add a new language but see untranslated text:
+
+1. Check if translation files exist in `src/backend/locale/<language_code>/LC_MESSAGES/`
+2. Run Django's `makemessages` and `compilemessages` commands to generate/update translations
+3. Verify frontend translation files are available
+
+## Related Configuration
+
+- `LANGUAGE_CODE`: Default language code (default: `en-us`)
+- `LANGUAGE_COOKIE_NAME`: Cookie name for storing user language preference (default: `docs_language`)
+- `LANGUAGE_COOKIE_PATH`: Cookie path (default: `/`)
+