If I want to translate a mod to Brazilian portuguese, do I need to do anything besides translating the "english.xml" to "PortugueseBrazil.xml"?
Furthermore, if I want to offer myself to translate mods made by other people (again, to PT-BR) what's the best way to do it? Do I PM individual modders about translating their work? Do I make a post in one of the boards? If yes, which one? Or am I blind and there is a better way to do it?
You should either pm the modder or post in the respective forum thread. You should find those under "Releases" which is a subforum of the mods section.
I guess most if not all modders will be happy to include any translation offered.