I have a few friends from Quebec on facebook and they speak solely in French... and I know very little French. If a status or image comes up that I'm interested in seeing what they say, facebook conveniently puts a "translate" link underneath their post.... and bam - it's in English. So I don't think your German friends should be offended if you're posting in English....... I'm assuming they have the same "translate" feature that should show up under your posts to them that they can click on as well... and it's their fault if they choose not to use it.

Speak whatever language you want to!! It's just facebook As mentioned, if they REALLY want to know, there are plenty of translation tools available, not to mention the built-in one on facebook itself.