The best way to find out what your brain does with languages is to try to pay attention to your dreams. It's hard to do that, but the few times I managed that (

The best way to find out what your brain does with languages is to try to pay attention to your dreams. It's hard to do that, but the few times I managed that (not on purpose), I realized that I would switch languages multiple times within the same sentence and it all "made sense", because my brain was picking the language that has the best fit for the concept it was trying to express. Of course, waking life is more structured than that, but I find myself struggling with the same problem there: I want to express something that's best said in one language, but unfortunately I'm speaking to someone in another language and now I have to pause to look up the best translation. Some things do seem to be subconsciously fixed, though. I find that when I have to count, I almost always do that in Serbian, my native language. Cussing, on the other hand, is something that I tend to do mostly in English, even when it's only in my head. That's probably because of my temperament, which makes me mentally cuss a lot. When I'm in need of serious cussing power, I switch to Spanish.