Jim Carroll and Joe Offer, my head's not on entirely straight today but the term 'Ballad English' isn't mine -- problem is that I can't remember where it originated! (My notion is Hamish Henderson, perhaps?)

Anyway, my (possibly limited) understanding of it is that the really big ballad stories -- of murder (Son David/Edward); infanticide (Cruel Mother); incest (Lizzie Wan) etc. etc. -- have more important things to say than identifying themselves as particular to a certain locale by the use of specifically dialect words, and so they usually present themselves in fairly unadorned English which is then given its local character by vowel sounds.
From my (admittedly small) experience of singing to American audiences, it seemed to me that they could follow the narrative if words were enunciated 'tidily', and they could then cope with my Scottish vowels.

IMO it would be a lovely scene if people felt comfortable singing in their own voice.

(And am I allowed to say that it would also be lovely if people could be persuaded to sing a more accessible/basic tune than the equivalent of a Mariah Carey version?)