Ahem: Hayakawa are slow. I think there's a lag of 2-3 years between them acquiring the rights to my books and actually translating and publishing them.

(It's worth the wait, but don't expect to see any action this year.)

One thing you should do is to politely prod your agent or whoever sold the Japanese rights and emphasize that you'd be delighted to assist the honourable translator with any issues of local idiom or vernacular they may encounter. (Because they may be too embarrassed to implicitly admit their own inadequacy by approaching you uninvited if they have a query ...)