📝(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

docs/languages-configuration.md

@@ -0,0 +1,174 @@
+# 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.
+
+
+
+### 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: `/`)
+