Need a guide for adding new languages

## 📖 Missing or Out-of-Date Documentation

### Problem
I would like to translate the website into other languages. However, it seems that there isn't a guide about the procedure of translation. It would be helpful for translators if there is a documentation covers:
- How to add a language for the website (by running a script or simply duplicate the `messages.po` and start modifying?)
- How to add an entry to the language selector
- Which file should be translated
- Some conventions and rules to follow
- Some explanation about the directory structure of the repository

### Location
/contributing/translation.md

### Content type
Contribution guide