Professor/supervisor switches from writing emails in his native and local language to English - how to react? A professor of mine, who's native language is German and who is teaching a class in English in a German speaking country, suddenly answered an e-mail which I wrote in German (I'm a native German speaker too), in English. I'm wondering why he does that? What the proper way for me to react is, i.e., should I use English when sending him e-mails from now on?
I wouldn't worry about it too much: given the bilingual nature of his work, your professor probably context-switches back and forth between languages frequently.  If the professor is very comfortable in both languages, they might not have even realized the switch, e.g., if responding to your email in the middle of a large block of work in English with their head in "English-mode".

If you want, however, next time you see the professor in person, you might ask if they have any preference for language in their communications with you: it could easily go either way (e.g., in English to make forwarding to non-German speakers easier, in German for personal comfort, or even no preference at all).