I was trying to edit some mp3 tags, when I noticed that the comment field isn't updated in windows explorer, although there were no errors in MP3Tag. Every other field went fine... I tried tag names such as %comment% and %description%, but no luck... Right-clicking the song in MP3Tag does show the comment, but in Windows Explorer, this information seems not to be there in any field...
With the latest Development Build, Mp3tag now has a new configuration dialog Options > Tags > Advanced which allows for the configuration of the ID3v2 comment language.
This implies, that setting the ISO language code via the language file is not needed (and not possible) anymore.