Should I write a bilingual email to a professor who can't speak German very well? Should I write a bilingual email to a professor who can't speak german very well?


That's basically the question. I don't know if this is the right thing to do, because it could mean that I underestimate his abilities.
English is the language of academia, so write in English. This is especially the case if his primary language is English.

In the reverse scenario where the professor is not very fluent at English but is great at German, still write in English, but feel free to sprinkle some German (e.g. in the salutation or signature). If he responds with more German, then you can use more German too.