I feel like this is an underappreciated fact: the American South is not the only place where people say y’all. My partner’s Indian South African relatives use it just as extensively (although they usually spell it your’ll!). Apparently it evolved separately in the two dialects.
@MikeJones Oh, for sure! With people more mobile than they were generations ago, and having access to mass media and the internet now as well, we do see this kind of cross-pollination between dialects. Apparently that's not how it developed in South Africa, though.