Despite the fact that I am a somewhat verbally-oriented person (I make my living as a technical writer), and despite the fact that I've always been a better singer than instrumentalist, I have no problem with my occasional failure to understand all the words of a song.

Also, some of my favorite songs are ones with minimal, nonsensical, or even "scat" lyrics, for reasons that I don't completely understand myself. Part of the reason may be that I've come to regard my own voice more and more as an instrument, not unlike a horn, and by extension to listen to the voices of others in the same way.

I suppose that I am one of those people guilty of greater interest in the "sound" of a song than in its meaning. I do prefer songs sung in English (the only language I understand), and I dislike opera singing as much as the next guy, for the same reasons as Dave-O (with whom I am expressing a degree of disagreement here). But as long as I understand the basic gist of a song, as expressed in the chorus/refrain or in a repeated title-line, I'm not worried about understanding every single word. In fact, there are songs I recognize and enjoy for years without ever clearly hearing and understanding every word, or even every verse.

And, to get back to the original topic, part of the "sound" of a song ~ which to me is more intrinsic and more important than every detail of its literal meaning ~ is the "accent" or inflection or tone-of-voice in which it is sung. So yes, as far as I'm concerned, songs from a given tradition should be performed with an intelligent and tasteful approximation of the appropriate vocal accent, be it Irish, African-American, Cockney, or whatever. It's up to the individual artist to exercise the proper restraint and avoid cariacture, self-parody, etc.