Thanks, that's something I've been wanting to tackle for a long time. I have contacted the Tapatalk staff about the issue.
On our other vBulletin-based forum we use UTF-8 encoding and there are no problems whatsoever with Japanese characters on Tapatalk. Unfortunately, it is - at least according to my limited knowledge - close to impossible to change the encoding on JREF without turning most of our Japanese threads and posts into mojibake.