The thing about being bilingual is mainly mixing the two languages, whenever the situation deems it fit, particularly if your character is surrounded by others who speak the same languages. In a casual situation, this applies particularly well when a character is trying to get a point across, but either lacks or has forgotten some word or phrase in language A, so instead opts to fill it in with language B. If your character, however, is the only one who speaks French, there are a number of common ways it can be used. First and foremost, my favorite, when your second language (though usually your mother tongue) is used for comedic purposes, be it colorful swearwords or frustrated blabber. This is something I personally do with my own friends, as most of us understand both English - my second language - and Swedish - my mother tongue, we tend to cuss shamelessly and go on aggressive rants in Swedish, for, when you scold or whine or cuss in your native language, it adds something more to what you're saying - sass, attitude, if used in the right way. I strongly suggest you do some deep research into typical French phrases/expressions, swears, words, etc. to make your character believable, and if you can, definitely try to talk to someone who's, like, properly French who can help you. Like some other comments mentioned, your mother tongue is something you instinctively come back to whether you like it or not, especially when it comes to counting (I count in Swedish but I think in English), or when you're put in an unexpected situation, something that might cause extreme fear, for example. Your character will likely also use their bilingual skills to their advantage when it comes to secret insults or details that they don't want other characters to understand/hear. It is also easier for multilingual people to make linguistic connections, for example, as an English, Swedish and Spanish speaker, I see words in other languages which I can relate back to what I already know; this is particularly convenient for written ciphers, clues, etc., but also just a generally good problem-solving tool when it comes to language barriers. If you know two languages, it is almost always easier to learn more, for example, as someone who speaks Swedish and Spanish, I understand both Danish and Norweigan, some German and French, but you get the point: languages are similar. You will, most definitely also make grammatical mistakes in the language you're less dominant in, as rules vary from language to language. One last thing I'd like to leave you with before I end this ridiculously long post is: sometimes, when a bilingual person is surrounded by people who only speak one of the languages they do, it can definitely be harder to communicate. As an example: I'm better at English than Swedish, so, when I'm around someone or some people that only speak Swedish, I tend to be more quiet, as it takes longer for me to find the right words and sometimes even the correct grammar, and expressing myself the same way I do in English as in Swedish is something I can really struggle with. Hope this helped, sorry for the information dump, I'm very passionate about these things. Good luck writing!