Stop making your multilingual characters “forget to switch back”, that’s unrealistic, here’s some actual situations multilingual people regularly do, as someone who is fluent in English, and conversationally fluent in Irish and Spanish:
If they have to say something in another language, like a name, it will be heavily accented, they won’t say it like it’s English
Multilingual people might have a hard time distinguishing languages that arent their first, and may mix them together in sentences
Translating expression directly that only make sense in one language
Adding sounds to English words because they don’t exist in their language (Eg: ‘Eschool’ in Spanish speakers instead of ‘school’)
If your character is a poet or author, consider the connotations of that language in written form. EG: Irish is a highly poetic language, and lends itself better to verse than English, so I would prefer to write more emotional things in Irish.
Not every word can be directly translated. Some concepts may exist in one language that don’t in English. This is a really good concept to use to make sure you’re actually exploring a characters culture, and not just making them multilingual for sex appeal or diversity points.
People who speak minority, oppressed languages will be defensive of them, and will meet bigotry against their language.
