We have a detailed page on how to contribute to the project:

https://symfony.com/doc/3.4/contributing/index.html

However, I think this page is missing a section on how to contribute translations, specificly:

• Where to look for translation files (Form, Security, Validator)
• How to determine which translations are missing.
• How to add them to a translation file.
• We always translate on the oldest LTS branch (currently 3.4 4.4)