That does, the trick, thanks!
My French books were declared with english language, which explains why the lookup in a French dictionary didn't work. By fooling around with mobi2mobi I set my books to neutral. If I'm not too lazy I'll try to set them to French (since the book I'm reading is _long_ hopefully I'll find the strength to do this before I finish it
Now I wonder if it's possible to set the dictionary language to neutral, maybe that would be simpler than changing the books...