...And I don't mean American English, in this case - I mean that American English speakers don't understand other-side-of-the-Atlantic English as spoken by regular people.
In support of my position, I give as examples:
1) The 1968 movie "Yellow Submarine" had non-Beatles playing the Beatles so their spoken dialogue would be understood by the audience.
2) Guy Ritchie's 1998 film "Lock, Stock and Two Smoking Barrels" needs subtitles in at least one scene when shown to an American audience.
3) In the Dreamworks animated feature Monsters vs Aliens, dozens of people on the internet have to misquote Gallaxhar's best fake swear word as "Flagnog" or "Flagnnon" because they don't correct for the slightly upper-class fake English accent and spell it "Flagnard" like they should.
OK, so I checked the subtitles. I'm like that.
Now that I think of it, maybe Americans don't even understand regular American English. On the DVD of Cloudy with a Chance of Meatballs, after you hit "play" but before the feature starts, there is a screen where they have two different stills of the movie that you pick one of. There's no explanation, just the two pictures that you choose one of. If you're watching the DVD on an old standard TV, you will probably think that you're choosing between full frame and widescreen. If you're watching the DVD on a widescreen TV, it looks like you're choosing between 1.78:1 and 2.35:1. As it turns out, it's the second one. Are we really there yet? Is everybody watching movies on widescreen TV's now? Are movie watchers afraid of math? Would have killed them to put a little bit of explanatory text on the screen, or does every interface have to be only pictures now? Do you understand the words that are coming out of my mouth?