So as someone who first language is English. How do you write dialogue that for a character whose first language isn't English. Because I tried doing this the other day and realized really quickly that I have no idea how to write someone who doesn't understand English as someone who has spoken it sense birth. (I also don't understand English but I can speak it fluently)

Have you seen r/writestreakEn ? It's a subreddit for English learners to practice and get corrections. (I use writeStreakCn to practice mandarin myself). Maybe it would be a good place to study how language learners express themselves?