It makes it easier for the audience generally for them to be in English. Vikings does it pretty well; everyone speaks in English, unless it's a scene where Vikings are talking to/around the English, at which point they revert to their native language to emphasize that, no, the two cannot actually speak to each other (unless one's learned the other's language). I feel like TMITHT does the German/Japanese as means of conveying emphasis; if it's said in Japanese/German, they are trying to be secret, are extremely upset, or something along those lines.