As a Geordie, I'd have little hesitation in attempting most songs from Scotland or Ireland- I'd draw the line at bothy ballads and the Freedom come allye' where there are a lot of Scots language words- likewise in Irish songs, the Irish/Scots Gaelic songs being out of bounds completely. I'd be much more wary of songs from southern parts of England, and I am sure southern singers are aware of similar difficulties, which are not just of language but of culture.
A song I've sung in the south is Ed Pickford's anthem to old age- 'Nee bliddy good gettin' aad'- now that is fairly easily translatable, and I have given the words to several singers in the south- (Ed loves his songs to be sung!)but the rhyming is difficult for non- Northeasteners- examples being

Your pub's never hord of a domino board and
Aa think that ye'll find that ye can't get yer wind (short 'i' sound'

these are Shiney Row rhymes and are hard to translate to what is laughingly called Standard English (there's no such thing!