Please check your settings in File -> Options -> Tag -> Mpeg - Write:
From the documentation:
- ID3v2.4 UTF-8 The newest revision of the ID3v2 standard. Tags are written in Unicode as UTF-8 encoded string. If you have problems with displaying the tags in other programs, please ask their authors to support ID3v2.4 (or switch to another option).
- ID3v2.3 UTF-16 If enabled, Mp3tag writes ID3v2.3 tags in Unicode encoded as UTF-16 LE.
- ID3v2.3 ISO-8859-1 If enabled, Mp3tag doesn’t use Unicode when writing ID3v2 tags. This is useful if you are using software or devices (like some car radios) which can’t read tags stored in Unicode format.
Unfortunately, if you haven't written your Japanese text in UTF-8 or UTF-16 from the outset, I don't see how you can translate/convert it later.
Could you show us a screenshot of what the text looks like now?