I have always thought that all the characters speak the same language, with only different dialect variations. In this case, what language or dialect would they be speaking? I also noticed that their "English" sounds even more broken than the rest of the cast.

In an interstellar community, single or more polities, there is bound to be linguistic drift, especially along lines of culture, religion, species, and to lesser extent environment of one's homeworld.

I've always thought of them as being from a very large family/clan with traditions of teaching the Family language first. You tend to speak with what you grow up with. especially to Family.