"What about some Gullah traditional songs, or songs in a completely different language, where if you sing a translated version into English or whatever your main spoken language is, it loses something?"

I think songs in a totally different language are less problematic. Because, er, it's a totally different language. So you sing them with the best accent you can muster.

But I can't think of any folk singers that deliberately chose to sing all/the majority of their repertoire in a foreign language. For the simple reason that it's a perverse amount of effort for not a lot of returns.

As far as translation goes, sure you do lose some nuances and of course the actual sound of the words changes, but I think of it as just being two different entities. Not necessarily worse. Just different. The meaning will inevitably change, but there's a reason why a particular person decides to translate a particular song (or poem or novel or whatever): usually it's because they feel some sympathy with the spirit of the original. A good translation sacrifices detail but maintains spirit.

I'm a big fan of Boris Vian, the late French surrealist, satirical singer and jazz trumpeter. Whenever I listen to his songs, I'm struck by how good the lyrics are and often think I might like to sing them. (I sing along in the privacy of my flat when I'm on my own.) I did French A-level, so I could probably make a reasonable fist of singing them in French. But ultimately, I reckon I'd be better off translating them and singing them in English. It'd feel more legitimate. His song 'Valse Jaune' (Yellow Waltz) is a prime candidate. Not that I'll ever get round to it.
(A Britfolk tribute to Boris Vian, anyone?)