That's an interesting question, which happened to come up at work the other day. I am trilingual, and I think in all languages. The deciding factor is what action I am doing or who I am talking to, in example if I am having a conversation with someone from Denmark, it just comes natural to think in Danish - otherwise the thought to speach process would simply be too slow in most cases. Especially at meetings or in conversations where you need to keep a professional image, then thinking in a different language than you're speaking can slow you down just enogh for you to seem uninterested or like you are stuttering and are unprepared. I am thankful that I didn't have my current job when I had just moved, because I wasn't mastering the language well enough to make it natural to think in it, meaning that I thought in Norwegian and spoke Danish-ish. When I use English-spoken forums or voice chats, I solely think in English. Sometimes I even catch myself thinking in English at other times where it would be natural to utilise Danish or Norwegian, but I take that as a good sign. I guess that's better than admitting to insanity anyway. ;) Off topic: I dream in all three languages, but mainly in the one I use the most. (Currently Danish.)