Now the above question may be relevant only to non-native English speakers who live abroad or to native English speakers who've lived in a non-English speaking place.
I've just come to realize that ever since I moved here, my brain had switched to English to think. Back in Indo, I mostly process my thoughts in Indonesian, though once in a while I forced my brain to think in English for practice sake. I only realized this a few days ago when I caught myself thinking in Indonesian. I felt that something was off, and then I shrieked inside and said, "Of course!!! I'd been thinking in English lately. No wonder it felt weird thinking in Indonesian again!"
So, how about you? I'm mostly curious about Max's answer, as she's lived in different countries (Portugal, Canada, London) and she can speak LOTS of different languages.