A person who takes over the stage at a folk club and sings a song people don't understand is sending the message "I don't care if YOU participate."

Well, that'd bugger up Sean Nos singing in Gaelic, wouldn't it? Or Klezmer, or... I agree, this kind of thing has to be done with some sensitivity. But that doesn't mean there should be some obligation to pretend English is the only langauge around. Which of course it isn't, even in Anglophone countries.