Transifex language files have already been uploaded. They just need contributors to start translating them. If you'd like to contribute to a language plugin check out localization page below for details. Your alternative is to use language overrides and translate strings there.
www.joomlapolis.com/localization
EDIT:
2) I tried to add a line e.g. in Religion, but when I try to save, I get an error notification: "Forbidden. You do not have permission to access this document."
this is only in some sites. I don't know what I need from my provider!
Sounds like a mod_security false positive from your host. You'll need to contact them regarding that.