The questions, answers and comments in this forum are mostly in English but there is some German, French, Dutch and Swedish as well. I would expect this trend to continue since geocachers are an international bunch. As for myself, I'm best at Swedish and English but will happily answer as best I can in somewhat butchered German as well.
As for the UI, I have switched it back and forth between English and German a couple of times and haven't seen any signs of the language changing when I don't want it to (apart from a few notification emails from this QA system that don't appear to be available in German, since I get them in English even though others are in German).
To determine what language you want Project-GC to speak to you, go to longW -> Settings and select the language you want. What language your local machine uses isn't relevant, as far as I know.