Dutch is my native tongue, so I think in that. I also think in English, part of this is the internet. Part of it is due to understanding most of CS and math in english terms.
I was alone on vacation in France, I learned some french in high-school. That combined with basically only speaking french to people there made me think in french a bit. Finally on that day, I had met a group of German scouts. I learned even less German in high-school, but it's closer to dutch.
So there I was, thinking in four languages. I recall at one point thinking of some English phrase and then thinking 'comment je le dire en allemande' which is my french attempt at 'how do I say this in German'.