Just a confirmation: I am a Japan resident, a subscriber to Amazon JP Prime and Music. Recently got my Wiim Pro, set it up, and logged into Amazon Music in the Wiim Home app. The log-in screen appears to connect to Amazon US but in fact, I was correctly logged into Amazon JP. No problems.
On a side note though, unless you can read Japanese, this will not be great for you. Non-Japanese artist names often appear in katakana (so, "Adele" becomes "アデレ"), and even the song titles, if the songs are very popular, can appear in katakana. I read Japanese just fine but I find this very annoying. Yes, my account is set to English language. Doesn't help. I think I'm done with Amazon Music anyway, and will shift to Tidal. Good riddance!