And to emphasize what every native English speaker says--and foreign students have trouble believing--it's one language. The differences are small. I (U.S. native) listen to BBC World Service effortlessly and with complete understanding. We can joke about differences, and misunderstandings, and you can find examples of rapidly spoken, colloquial British English that I'd have trouble understanding, but I'd also have trouble understanding many kinds of U.S. regional English. The first time our software QA head said "This needs fixed," I couldn't figure it out. I said "Uh... do you mean... this needs to be fixed?" And she laughed, and said, "Oh, sorry. I'm from Western Pennsylvania."
Actually just last month, just for fun, I posted examples of the same news story as reported in Australian, British, Canadian, and U.S. news and challenged people to identify which was which. Nobody could. See for yourself: