There’s an interesting master’s thesis for someone: how much difference in vocabulary is there between UK and US English, as opposed to regional differences within each country? My husband, for example, knows what lifts, lorries and bin liners are from watching Dr. Who and Monty Python, but when we moved to NYC, where he had never lived before, he occasionally encountered words he’d never heard before.
I don’t know what the UK term for a carbonated soft drink, like Coke, is, but around here, you might hear “soda,” “pop,” “soft drink,” “seltzer,” (although people who say this are dying out), “Coke” as a generic term (“What kind of Coke do you have?” “Pepsi, Dr. Pepper, 7-Up, Diet Pepsi”), and for trash cans, you might hear “waste can,” “waste basket,” “garbage can,” and a few more.
If you are going to call British English and American English two different languages, then you might have to call the English of East Texas and the English of Brooklyn two different languages.
The only time I couldn’t understand someone who was English, it was in Limehouse, and he was drunk and missing most of his teeth.
Now, it’s true that people do “code-switching,” so that they attempt to make themselves understood, and aside from the missing teeth, the guy in Limehouse may have been too drunk to pick up on me “not being from around there,” and make fine adjustments in his pronunciation and word choices, but I think just plain old drunk had more to do with it than anything.