I'm from Belgium too, Dutch-speaking as Jeroen is. I do recognize his story, got French at age 9 or 10, English at 13 or something and German for the three last years of high school. I think mainly in Dutch or in images actually, but indeed, when reading English course books or taking a class in English (I mean any subject, courses given in English) or talking to English people, I sometimes think in English, take notes in English etc. Same sort of applies to French, but that takes some more time: after working in a trilingual office during internship, my English and French were jumbling and tumbling around, over, under eacht other Having such a chance to practice is great though, my French-speaking colleague wanted to practice Dutch and I wanted to practice French, so we spoke each others language and corrected the other when necessary. Dutch is of course my favorite, as it is my mothertongue. I sometimes mix in some English if there's a nice way of expressing things, not present in Dutch.