Re "The Strawberry Roan" - Charles Parker was the 'guilty party'.
I can vaguely recall back in the 1950s on the BBC Home Service(!) there were programmes which included trained singers singing things like "Blaydon Races" in a strange melange of Geordie and plumminess!
Foreign languages....how many remember, or even sang "Everybody loves Saturday Night"? As I recall, it went through about ten languages.
If you're serious about singing in a foreign language I would suggest trying to learn the song(s) in question from a native speaker rather than from a book. There are so many pitfalls, subtleties and niceties in terms of pronunciation, fitting the words to the tune and so on that it is very easy to make a fool of yourself. (Yes, I know Pete Seeger gets/got away with it).
With accents and dialect, in the end it's probably a question of feeling comfortable with what you're singing. I've yet to encounter a pedant complaining that I've toned down a regional accent or dialect a tad. Give the song a few months to 'bed-in' before going public and, whether you've anglicized a word or two or come to an 'arrangement' with the dialect, it'll probably feel alright. It's probably best however to keep to accents and dialects within your comfort zone. I come from Kent and for me, singing something in Geordie, particularly the 'pitmatic' variety, like "Little Chance" would be way outside my comfort zone. On the other hand, I grew up in Derby so that if I sing songs I've written about Derbyshire I probably automatically take on a slightly East Midlands accent.........
"The Band O'Shearers"..How about this 'anglicization'?:
O bonnie lassie will you go
And shear with me the whole day through?
And love will cheer us as we go
To join the band o'shearers.