If you have any questions you can always reach me via email (giantpinkrobots@protonmail.com) or through Matrix (@giantpinkrobots:matrix.org). Thanks! :)
If you want to contribute code, you mainly want the 'next' branch for things like new features that'll land in the next major version, as that branch is for active development. For important bug fixes however, you want the 'main' branch, so I can release a hotfix quickly from there without having to also release all the untested new features in development. These fixes can be applied to the 'next' branch later.
In the case of translations, you want the 'next' branch unless if there is a really embarassing mistake that needs immediate correction with a hotfix, in which case you want the 'main' branch. Translations are done with .po files that you can create with a program like Poedit for example.