Interesting!
For me, French is the language I like to be negative in. It's not a language I'm native-level fluent in, but I'm at the level where I can carry out those mental self-conversations you have with yourself when thinking in French as well.
In french, negating a statement involves two words sandwiched around the verb (usually ne...pas, e.g. "il ne trouve pas"). I think for me this gives the negation a lot more "weight". It's hard to explain (and I'm not even sure if this is the explanation, it's more of a guess). But if I'm thinking a strongly negative statement I'll often think it in French.
There aren't that many things for which this happens for me, most kinda of thoughts get expressed in no particular one of the languages for me. But most strong negatives happen in French (and certain kinds of questions happen in my non-English native language).
I wonder if it's just a coincidence or something special about negative statements going on here.