I have a similar experiance. German is my mother tongue, but I learned English from an early age at bilingual primary school. I «think» in both as well, but depending on the situation it might also be just one or the other. I have started to observe when I think in one over the other and found it comes down to subject matter. American politics is english only as almost all media I consume on the subject is english. Law (my field of study) is German, since I am studing in German. For other things it seems more random.

Similarly I find that when writing (just for myself and can thus choose the language) I tend to use English very differently and in different situations from German. I also sometimes choose one for stylistic reasons. A manifesto for instance sounds a lot better in German (not that I have writen a manifesto). I also found that for many words the literal translation doesn't always capture the same nuances.

English seems like the more "frivolous" language to german. Maybe thats because german is my second language, but I doubt it. Big words don't seem as big when spoken in English as compared to German.

On my fathers side of the family there are a number of en/de bilingual speakers, so we also speak a wild mix of the two picking and choosing what fits best (or just popped into my head first).

This experiance has made me very interested in learning French as well. I speak it a bit at it's mandatory to learn french in school here, but I am still far from being at the stage where I don't have to mentally translate everything in my head and just think in the language. I am moving to the french speaking part of the country soon for that reason (among others).