Professor/supervisor switches from 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?