I think 'Suffolk Miracle' had it about right - naturalise it if you can, and if you can't then sing something else. I did once do horrible things to Twa Corbies, although in my defence

a) I only did it because I wanted to pair it with Three Ravens (is Twa Corbies an 'answer ballad'?)
b) I asked if there were any Scots in, and apologised to them in advance.

It's a challenge - and fun - to get inside the skin of a song in a different version of English (or a close relation of English, depending how you see Scots): those birds aren't crows and they're not cawbies either, they're corbies with an R, and they're not "making moan", they're quite definitely makkin' mane... But I think the temptation should be resisted - it's not so much a foreign language as somebody else's language, and in the case of Scots or Irish English that somebody might well be in the audience.