I can hear myself thinking in a very specific patois when I'm writing fic for Kaminey and I write English language fanfic since the framing voice, which is mine, chooses my own English idiom. I work on getting the same sound in English. I try not to typecast characters by correlating their voice to a similar voice in English, since I abhor reading, say, Cockney slang from some charming scoundrel who is clearly speaking in a whole different language. Instead, I try and focus on the lower registers, under the tics and slang, and try to mimic the way the character would sound to herself or himself, in their own head.

I usually write in third person, so that helps. And when I need to use a word from outside English, I just do, and provide a glossary at the end.