Enable translations in gui apps?

Help
1

I set the value for i18n.defaultLocale and saw some apps with localized text in their UIs. What else do I have to do to have more translation coverage? Is there a way to enforce the language for firefox and libreoffice for the whole system so each user won’t have to go to these apps’ settings to set the language?

I don’t know much of how translation is handled in linux and nixos in particular so links to articles covering the topic is welcome.

2

also, I may have some translation that is more updated than upstream’s version. How to I integrate that to a nixos system without having the package compiled from scratch?