I'm not exactly sure how to rate the fluency of your own native language, especially since usually, fluent is used as a sort of a level under the native speaker level?
But, I'd say my english is more polite than my French, I do use quite a few swear words, and my usual French is slightly working class, I can switch to formal and use big words, I just don't want to use my energy to do that most of the time.
However, even taking into account my working class roots, with the swear words, you can notice pretty easily that I have more vocabulary, and uh, I make a lot less grammar mistakes than the average population when writing. I do pale in comparison of Academics, it's easy to notice that I don't come from money (as in, the only other person who went to university, in my family, for now, is my mother's cousin), if you've been to uni, or are generally rich.
I have a slightly southern accent that tends to come out stronger the longer I've been speaking in English, or if I talk to people from the north of France