I'm not a english native speaker, and I make a lot of grammar mistakes although I study english since elementary school lol. Written communication is waaay moooore simple than verbal communication. You can take your time to think, while during conversation you can't. But when you keep practicing, you'll be fluent in both type of communications. And that's what Ayu will do
as other said, Ayu knows english well. Would be more diffucult for Mannie learn japanese rather than Ayu learn english imo.