that is not a thread, and it was 2009
as a Swiss German with Italian, Spanish and French parents my music collection is rather multilingual as you might expect. still, 65% is English, just like the volume of music in the music market. if a spell checker cant auto-detect the language, assume it's English and manually change if it's otherwise.
I would apply the spell check on album titles and song titles.
exceptions like the Slade one would be treated as such. the vast majority of song titles and album names are not slang. apart from the rnb and hiphoprap culture. I agree, there this could be quite pointless.
it's just sad how many English natives dont get that right, at all.
the files I process are btw from officially licensed sources. I mention this because even from the source, apparently it seems quite impossible to spell check the crap before it leaves for distribution.
for now, I use my renamer to do the spell checking but I'm a big fan of mp3tag and maybe some day, this is another feature of it.