Since version 2.33 Mp3tag supports Unicode through the whole program. Mp3tag writes ID3v2 tags in Unicode format (UTF-16) by default now, but there are still some programs, portables or car radios which have problems with ID3v2 tags stored in Unicode format (though this is specified within the ID3v2 standard).
To use Mp3tag with these programs or devices, simply enable the option
(x) ID3v2.3 ISO-8859-1 at Options > Tags > Mpeg.
After that, just save the tags again by clicking or by using the [Ctrl+S] hotkey.