I've read that Ewan MacColl at his folk club enforced a policy that no one could sing a song in a language they didn't actually speak. Apparently this caused some controversy. There might be something to that, though. When songs are sung in English by people who obviously have no fluency in the language, they tend to sound obviously stilted.

Can a fluent speaker of Irish or Scots Gaelic usually tell when someone is faking the language (i.e. singing from a syllable representation of the lyrics)in a song?

Jon Corelis
Celtic suite in G major