Tips for writing multilingual characters:
If English is their second language, they will speak quite formally if they’re not entirely fluent. Contractions are a B2-C1 level type of thing for most. It also depends on the cultural context of where they are from. Some cultures place a higher emphasis on good spoken English.
Look for features in their first language and make them say them in English. Russian, for example, has no articles (the and a), so Russian speakers commonly struggle with articles.
Pronunciation is a really big one. For example; first language Spanish speakers would often say “espain” because Spanish rarely has “s” without “e” at the beginning.
Names, places and things in their first language will be heavily accented, and they will not say them like an English speaker
NOT ALL LANGUAGES HAVE THE SAME ALPHABET!!!! Many languages lack certain sounds that English has. If you reslly want to be as realistic as possible, find the youngest shared ancestor for English and their first language, and find the sound that language took instead of whatever sound it became in English. EG: Italian lacks the letter J, it became I. So Italians may pronounce “J” (as in “jug”) like a “Y” (as in “youth”)
Remember that English is not a neutral thing for a lot of people, and speaking it comes with a lot of pain. Cultural context is deeply important to your characters relationship with English as a language.
It does take a lot of mental energy to translate everything you think. If a character is not completely fluent, in states of extreme tiredness, injury or emotional distress, they may struggle more with English
The time taken to learn a language is not universal. Languages are grouped, and it is far easier to learn a language from that group than one that is not. It also depends on the amount of practice, exposure to the Target language and location
