People who speak more than one language aren’t as likely to accidentally start speaking another language as they are made out to be. There are little slip-ups, however, that are more realistic. In my experience, this is what I think it’s more like when you know/have learned/are learning multiple languages:

Your character might use the wrong word when listing things! I’m very likely to sometimes use the wrong word for an item on my grocery list. Even if it’s just one or two items randomly on the list that are in another language, and I might not notice them until later or if someone points them out. Sometimes I’ll put the first letter, realize that I’m about to write the wrong word, and come up with something else I need that starts with the letter I accidentally wrote.

Sometimes sentences come out so wrong, but not necessarily because of a different learned language. More like it’s out of normal English word order and it sounds ridiculous. I obviously correct myself as soon as it happens and make a joke usually, but that happens a lot if I’m not thinking while talking or if I’m talking too fast. I get really embarrassed about it, especially because I’ve been around English for nineteen years lmao but not everyone does- and they shouldn’t be because they know more than one language and that’s so cool!

I forget words ALL the time. I will sit there and try and describe it or be like “the thingy”. Sometimes it will seem so obvious and it could literally be the dumbest thing, but I think it’s much more likely than just ‘switching’ into the wrong language for other people as well.

Think about your character’s problem words! I have a tendency to have to ask for confirmation on certain words, avoid using certain words, or think I’m spelling words incorrectly because of the way they look/sound seeming wrong to me. For example, I had to type ‘usually’ twice in the second bullet because it looked wrong. I have issues with other words like ceiling, separate, recommendation, etc. Sometimes when I write them out or type them they look wrong. I’ve also been told I pronounce certain words like ‘both’ and ‘tourist’ weirdly?? 

If one of your characters knows sign language, they might have a tendency to use their hands when talking, signing words they’re saying with small motions in front of their stomach, near hips, etc. Or some specific words they may sign while speaking frequently. For example, I have a tendency of using the “same” or “me too” sign a LOT while talking, more frequently than some other words I sometimes don’t realize I’m signing. 

Sometimes with fatigue or caffeine-deprivation, I will accidentally almost start a sentence in the wrong language BUT I notice. I don’t sit there and try to talk to someone in the language not noticing, it’s more like I almost do and it’s a very quick ‘no, wrong’ in my brain.

I don’t experience this, but sometimes people learn certain subjects in certain languages, so they think about the subject in that language. Someone might learn math in their native language and never learn it in English, or astrology in a certain language, or maybe just science in general. Maybe they analyze literature in their native language. 

Sometimes people speak a ‘franken-language’. My old art teacher married a man from Colombia and she speaks ‘Spanglish’ lmao. She knows a lot about Spanish, a lot of vocabulary, and they spend most of the year in Colombia, but she’s not fluent so sometimes her conversations are more like a bunch of parts from both languages put together.