Hello,
The language of Mp3Tag can be changed via "Options" > "Language", selecting your desired option and submitting via "OK". This will prompt a restart of the application to apply the new language.
However, by just selecting a language -without pressing "OK"- and then selecting another option ("General", "Messages", etc.), Mp3Tag behaves as if "OK" had been pressed, by showing the restart prompt - now in the new selected language. Choosing "No" (do not restart) closes the pop-up, but now the right-most pane of the "Options" window displays the new language in any of the major options, except "Language" and "Updates".
Exiting (via escape, via close icon) and re-entering "Options" will now show all panes in the new language. Other menus show fields in the new language. Re-starting Mp3Tag will fully load the new language.
Also, some language flag icons seem to be missing (verified on portable installation).
I have been able to verify this behavior as early as v3.19 (64bit).