Some of mp3 files have only ID3v1 tags and these tags may contain international characters.
In this case mp3tag shows garbage instead of text. Please, look as screenshot.
You see how mp3tag shows text on the left of the screenshot. And you see Windows Explorer property sheet for the same file on the right.
Windows property sheet displays correct text. I have marked invalid mp3tag counterparts on the screenshot.
I guess, mp3tag assumes iso-8859-1 encoding for ID3v1 text tags, but, in this case the encoding is windows-1251.