There are certain singers who are actors. Jack Elliott is one. They have the gift of mimicry.
Frank Warner was another. Ewan McColl. The New Lost City Ramblers. Their ears pick up the speech patterns and accents of others successfully for most.

There are plenty who don't. Pete Seeger for one. He sings the way he speaks.

It comes down to which interpretation of a song you accept. There are those who go
to concerts of classical music and follow the score to intercept the mistakes. These folks exist in the folkie world too. They are more focussed on the accents then the content or performance of the song.

Folk music performance is often the presentation which falls into the category of
stagecraft. If the audience believes the performer is "real" than it makes no difference
to most what accent they acquire or affect.