Writing in the New York Times, David Cohen meditates lyrically upon the differences between British and American English. Cohen uses these differences, especially individual words--loo vs. bathroom, bonnet vs. hood, car park vs. parking lot, etc., as a token of a larger cultural divide. He quotes Victor Katz, noting

There is the illusion that we speak the same language, but we really don't.

And then Cohen concurs, adding what might be a passing dig at supposed American corruptions of the language:

Yes, the illusion is there. The United States freed itself from Britain in a revolution but had to opt for subtler forms of sedition when it came to the language.

To call the notion of a common language between Britain and America illusory, an idea that goes back at least to Shaw's quip that "England and America are two countries divided by a common language," is in one sense too strong, and in another, not strong enough. Cohen's strategy for revealing the illusion is revealing. His markers of cultural differences are individual words that are essentially arbitrary. There's not much that one can infer about how two cultures differ from the fact that one calls the piece of glass in an automobile that protects the driver from wind a windscreen and the other calls it a windshield. The specific words don't refer to anything of cultural note. They are arbitrary signifiers, and what they signify is irrelevant. In short, they announce, but do not describe, a difference--a very Saussurian way of looking at language. The differences in word choice between American English and British English boil down to vocabulary knowledge, which doesn't just differ from region to region or group to group, but from person to person. If you're American, it's statistically likely that you don't know what chav means, and even more likely that you wouldn't use it in your own speech. But what about palimpsest, or quaquaversal, or eleemosynary, or whilom. Do you know them? Do you use them? I can't predict whether or not you will know those words based on your nationality, but I can confidently assert that most Americans don't know them. I however, am an American, and I've had recent occasion to use all of then. Does that mean that the notion I share a common language with other Americans is illusory? It is in the sense that language as an external, reified thing--Saussure's langue--is an illusion. There is no language "out there" in the world apart from individual human beings. It is all "in here," in each of us. We are language, and each of us instantiates it in a slightly different way. And so none of us share the exactly the same language, but our languages overlap sufficiently that we can still communicate. The illusion (or if you prefer, convenient abstraction) is more intense the more our languages resemble one another's, but it remains an illusion. For that reason, I find Cohen's conclusion off the mark:

In fact, the world's lingua franca is now bad English. It's strange then that a U.S. president who speaks good English, far better than his predecessor, seems able to communicate with that world. This may even be Barack Obama's biggest achievement in his first 100 days.

Cohen purports to find Obama's skill as a communicator "strange," but it's only strange if you take the reification of language to a ridiculous, almost Platonic extreme. The observation about English as a lingua franca alludes to the way that people whose native languages aren't English are using and adapting the language to communicate with others whose first language also probably is not English. Cohen treats this "bad English" as an external entity, a quasi-standard form to which one must conform to be understood. Perhaps it should be capitalized like other varieties: He speaks Bad English. But the abstractions of British English and American English have better justification than Bad English. At least they are a shorthand way of referring to coherent bundles of features shared by speakers in a particular region. There is no such unifying feature for Bad English, particularly if we are to lump George W. Bush in with all the ESL speakers. Hat tip to Mayank Keshaviah