The general litmus test for whether 2 people are speaking different languages is whether or not they can understand each other without taking classes.
I don't have to take UKEnglish101 in order to understand that a rubber in england has nothing to do with sex. And some guy in England doesn't need to take USAEnglish101 to understand that a pair of boots does not require one to have a mid-engined car
If swapping out a few words were all it took to make a seperate language, then there would be Wisconsinish, because for some reason the weirdos around the Green Bay area have been known to call water fountains "bubblers."
Oh, and "ebonics" is just a pathetic attempt to legitimize crappy english skills.