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.
-Related to that second to last one, when we’re talking with other people who are the same kind of bilingual we mix words from the other language into our sentences. I notice that my colleagues, who have the same native language and second language as me but are more assimilated in the country where my second language is spoken, do it more than me but I still do it sometimes.
-I actually have forgotten to switch but it was only when I was really caught off guard, think saying “Fuck!” in a situation where I would normally speak my second language. It should be noted that, like I said above, I was never particularly assimilated in the country in question and toward the end I said “mood” when Louise Watts cried about wanting to go home. Figure out how assimilated your character is in the society in question and ask people for advice based on that.
Also, from someone that use at least 2 languages everyday:
forgetting in what language you read something and having to explain you're not trying to push a foreign language on people when you have to backpedal because "I forgot the book was in English and you don't read books in English, sorry"
Even if your character isn't a poet, chances are they will find it easier to speak about certain topics in one particular language either because they read/studied mostly about it in one language or because they associate certain emotions to one of their languages. I know someone that will seem stoic and at times cold in a language but will open up far more easily in another language for example because talking about their emotions feel easier in one language.
The utter doom one character can feel about being extremely smart in one language but barely able to read a book in another of their languages even if they're able to hold day-to-day conversations can lead to some characters getting frustrated and show there is more to them than what was believed in a monolingual context.
For characters that often switch between languages enough or are polyglots, it's possible - even if relatively rare - they need a moment to remember a word... from their native language. It's extremely frustrating to wait for your neurons to connect again because you remember the word in several other languages but can't remember the actual word in your own native language.
