If I may insert another angle to this discussion:   

There is a reason that Italian opera is almost always sung in Italian and French opera is almost always sung in French and German opera is almost always sung in German, even when the singers are not Italian or French or German. The text was written in that language and the music for it was composed around the characteristic rhythms and inflections of that language. I have heard operas translated into English from their original language, and it always loses something. It simply sings better in the original language. I don't see that folk songs and ballads are any different in that respect.

[Note:   FYI, modern opera houses have a feature called "supra-titles;"   there is a long, narrow, flat panel above above the stage where they project an English translation of what is being sung—very much like the subtitles at a foreign movie—so you know what all the singing is about.]

No one, including Italians, object or are offended when Americans like Jerry Hadley (tenor) or Marilyn Horne (mezzo-soprano) sing Verdi or Puccini in Italian, or Australian soprano Joan Sutherland sings Donizetti. Can you imagine someone trying to sing a Gilbert and Sullivan comic opera in Italian? And how much of the humor in the "patter songs" would be lost (much of it depends on goofy rhymes) because of the change of language?

The same idea pertains. There may be huge stylistic differences between opera and folk music, but in any form of song, the way words and melody fit and flow together is important and has a lot to do with the song as a whole. Switching languages or altering dialects, even changing a word or two (unless it is done very thoughtfully and carefully) can be like trying to stuff a size eight foot into a size six shoe or vice versa.   

Jerry Hadley and Marilyn Horne and Joan Sutherland learned how to pronounce the languages they sing in as best they can, and they just haul off and sing. But they don't change their names to something Italian-sounding and try to convince people that they are Italian.

Don Firth