-About different languages. It shouldn't be a problem for my configuration as I have physically split my library by language. One folder for english songs, one folder for spanish songs, and other folders for best music in italian, french, catalan, german, and finally others.
-For artist field it would not help much.
-For title field of tracks it could be helpful, but maybe the last.fm spell checker is more convenient.
-For lyrics it would be useful but rarely I would use it. The first problem i see with it is the different formats, for example i use minilyrics with priority to .lrc files, later .txt and finally MB tries sometimes read a embedded. Here we see the convenience of having the lyric in a external file as maybe all of this would be as easy as sending it to an external editor and if possible passing to it the language in the command line.
While i see it helpful I give a +0 as i would use it rare times and have a workaround.
The workaround is using any text box in Firefox, for example i have enabled spell checking in english for this site, and spanish for facebook. We paste the lyric in that text box, fix it, and we copy and paste it to minilyrics.
Another thought is it could be really convenient sending this wish to the MiniLyrics developer