Off topic: Is it common practice for a dissertation at a German university to be written in English?
Yes - to ensure the widest possible audience. English is the lingua-franca of academia and has been for almost a century now.
At least in MINT fields, probably less so in the humanities. Correct me if I'm wrong.
You're right. German economics is famously insular, for example. Part of that is cultural, part of it is that they don't write as much in English.
Depends on the field, but it is very common in computer science and generally in natural sciences. Studying computer science, I don’t think I ever even wrote as much as a paper in German; everything’s English.
Hardly anyone is going to read, let alone cite your work if you were to publish in German. I'm not even aware of any German-speaking conferences.
In physics it certainly is. As far as I know, much all scientific conferences and journals are in English these days. Even journals like JETP (https://en.wikipedia.org/wiki/Journal_of_Experimental_and_Th...) became joint English and Russian in the 50's!
It is also common in many fields in Switzerland, Italy, Spain, ... Newton's Principia was written in Latin, for example and not in 18th century English.