Incredible as it may seem, this has not been discussed yet. This very old question deals with whether the title should be translated in the question’s body. This one and its follow-up only deal with the language of the actual question’s body. And this one maybe recommends editing to make a question one common language, but also mainly deals with question bodies.
This question is sparked by this one on the main site where the OP insisted on sticking with an English title for a question entirely in German.
In which language should the title of a question be written? Which reasons would there be for using a different language in the question’s title than in the body? Assume monolingual questions only when answering.