I'm not a native Japanese speaker, but I have been using AI to write 95% of each email in Japanese. I try to avoid using words which I don't understand.

I've gotten personal praise from many about how perfect my emails are, though the expectation is likely lower because I'm not Japanese.

I mostly use Claude for simple engineer-to-engineer communication, which has always sounded robotic, so I don't have much experience with long-form.