If the question was translated by the original author, i.e. the question was asked bilingually, I'd choose whichever seems more appropriate for the answer. If there's a lot of English involved due to the subject matter, it may be easier keep it all in English. All things equal, I'd lean towards German though.
Make sure there's always at least a German or English translation of your answer, should the answer be written in another language.
If the question was community-translated after the fact, answer in the original language as a courtesy to the author. If you're not confident enough to do so, answer in the language you feel comfortable with (given that's English or German) and perhaps leave a note soliciting a translation.