I don't speak the language, so they could have been speaking Mandarin, Cantonese, some other Oriental language, or Swahili, for that matter. I couldn't have been able to tell one way or the other. As for their pronunciation, however, it must be remembered that they wouldn't be speaking the Mandarin language as it is spoken today, but rather the way that it will be spoken several centuries in the future. Languages change due to any number of factors, and being separated by several light years from the "Earth that was" and it's actual source, there's no possible way how we could conjecture on their pronunciation by that time. The fact that we can actually understand the English language is probably the most unbelievable single element, though an inescapable one, of the entire series.
Back during the "Enterprise's" original five year mission, I remember Gene Roddenberry addressing this as regard to the language, slang, customs (The Federation celebrates Thanksgiving with the traditional turkey dinner), and general expressions used by Kirk and his crew by simply saying that it would be impossible to present the show any other way without using subtitles for the entirety of each episode, something that the fans wouldn't care for. I think that that pretty much applies here as well.