Singing a song in a language which is foreign to you, and which you do not understand, depends very much on the context in which you do it; done "for a laugh" in an informal situation, I'm sure it's harmless enough.  Done professionally or semi-professionally in front of an audience in a relatively formal situation, you certainly risk giving offence or making yourself look stupid.  Each to his or her own; I am comfortable singing in French, because I know what I'm doing, whereas I do not sing in Gaelic, however much I might like to, because I know very well that I could not do it properly.

I believe that it's all about showing proper respect to a tradition which is not your own.  If that makes me a "folk policeman", then so be it; my own feeling -and these things are always subjective- is that it simply reflects an awareness that music is not necessarily "common property", to which people can do whatever they like without always taking the trouble to find out what it means, but that it does have a meaning, and that the kind of instant gratification which so many people demand ("I can't be bothered finding out what this neat-sounding song actually means, so can you tell me how to pronounce it phonetically", etc.) can sometimes result not from a genuine love of music, but from a kind of acquisitiveness akin to cultural imperialism.

And before anybody says, "opera singers do it all the time"; that is exactly what I mean.