As someone with multiple bilingual friends where English is not the first language, may I present to you a list of actual incidents I have witnessed:

Forgot a word in Spanish, while speaking Spanish to me, but remembered it in English. Became weirdly quiet as they seemed to lose their entire sense of identity.

Used a literal translation of a Russian idiomatic expression while speaking English. He actually does this quite regularly, because he somehow genuinely forgets which idioms belong to which language. It usually takes a minute of everyone staring at him in confused silence before he says “….Ah….. that must be a Russian one then….”

Had to count backwards for something. Could not count backwards in English. Counted backwards in French under her breath until she got to the number she needed, and then translated it into English.

Meant to inform her (French) parents that bread in America is baked with a lot of preservatives. Her brain was still halfway in English Mode so she used the word “préservatifes.” Ended up shocking her parents with the knowledge that apparently, bread in America is full of condoms.

Defined a slang term for me……. with another slang term. In the same language. Which I do not speak.

Was talking to both me and his mother in English when his mother had to revert to Russian to ask him a question about a word. He said “I don’t know” and turned to me and asked “Is there an English equivalent for Нумизматический?” and it took him a solid minute to realize there was no way I would be able to answer that. Meanwhile his mom quietly chuckled behind his back.

Said an expression in English but with Spanish grammar, which turned “How stressful!” into “What stressing!”

Bilingual characters are great but if you’re going to use a linguistic blunder, you have to really understand what they actually blunder over. And it’s usually 10x funnier than “Ooops it’s hard to switch back.”