📝(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

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