Well, I speak 4 languages on a daily basis, which are Dutch, Turkish, English and French. The first three languages are languages I can 'feel' so to speak. Dutch and Turkish are my native languages and even though English isn't strictly my native language, I still regard it as such because I've started understanding, speaking and writing it from a very early age on and I can 'feel' it. I've thought about this question before and I can say that for me it depends on what I'm thinking about. If I associate a certain topic to say Dutch because that's the language I speak in about that particular topic, I find myself thinking in Dutch when I think about that topic. This also counts for 'monologues' in my head or when I find myself discussing with myself in my head, it changes. And I couldn't tell you in which language, it could be either Dutch, Turkish or English. I almost never think in French, unless I'm preparing a speech or a discussion with a French-speaker. One peculiar thing is that while performing basic arithmetics in my head, it's always in Turkish, for two reasons. First is, as opposed to Dutch (which is the language in which I've attended school which is what makes this interesting), all numbers are not only written but also pronounced from left to right, just like in English, so I prefer Turkish over Dutch. Secondly, and this is why I prefer Turkish over English, in Turkish most words are much shorter which helps speed things up (Thousand = Bin, Hundred = Yüz,..)

People who are bilingual are comfortable thinking in either language. Depending on the subject, or the people they are with, they may automatically start thinking in one language, rather than the other. Some of them may have a preference for one language over the other. Research has shown that bilingual people do not develop Alzheimer's disease as soon as monolingual people. So, be glad that you have 2 native languages :)

@Katherina Yes, that happens to me as well :D When I'm speaking to some of my closest friends, which also speak at least 2 languages, words in sentences get mixed up and it happens so fast and thoughtless that it sometimes amazes other people who happen to listen in, especially when we're in Turkey where most people only speak one language.

I know one girl, she's native Russian and German speaker. She can start a sentence in German and then suddenly continue it in Russian. It's because she thinks in both languages :) I don't know English well, but sometimes I think in English. For example, I don't need to translate words, which I know for a long time, I only need to translate new words. Or I can think in English after watching English TV Shows, when I heard English speech for an hour.

'..., IN which of THE two, DOES he/she think?' :-)