So, you want to learn English or at least learn it better. Even if you don’t, there is sure to be someone — a teacher, a spouse, a child, a boss — who thinks your life, your career prospects or even just your vacation options would be greatly enhanced if you did. No problem there, you think; Japan is chock-a-block with English-language instructors. So it is, but as any language student knows, and a book published in America last month confirms, the process may not be that simple. Before you can learn English, it helps to know what English is, and not even native speakers can agree on that.

In “Do You Speak American?” — the companion to a television documentary and sequel to the hugely popular “Story of English” (1986) — authors Robert MacNeil and William Cran share findings from a cross-country trip they took to determine the state of “American English” today — in effect, to find out whether there is, or ever was, such a thing. Some academics and language purists, the authors write, seem to believe that there once was, but that it is now in sad decline. Others argue that American English has always been a hodge-podge of regional and class-specific dialects, but worry that those might be vanishing under the pressure of cultural homogenization.

Unsurprisingly, Mr. MacNeil and Mr. Cran discovered that there is no simple response to either of these concerns. They did discover that quirky, regional English is flourishing across America, even though everyone in the country watches the same television programs, eats the same fast food and buys the same name-brand clothes.

But they also found that there is still a recognizable “American English”: Rather than being dissipated or destroyed by new words pushing up from below, such as “bling bling,” or flowing in from outside with the latest wave of immigrants, the language simply embraces them, as it always has.

American English exists, in short, but it is flexible and various rather than fixed or monolithic — no matter how much the purists may object. (“Do you speak American?” was the question put to the U.S. Federal Communications Commission by Bono, the Irish rock star, after it fined him for using a common profanity at last year’s Golden Globe awards ceremony. But it’s a question that applies to a lot more than profanity. Who, Bono was asking, gets to set the standards?)

And that is just American English. Globally, English exhibits the same qualities, and presents the same challenges, only on a larger scale. Pinning down a standard for the language is about as easy as catching a bead of mercury. Foreign students may actually be more aware of this than native speakers.

Take a person setting out to learn English in a giant metropolis like Tokyo. He or she can choose among many teachers, all native speakers but each offering a slightly different version of the language, complete with individual accents and even vocabularies: American, British, Australian, New Zealander, South African, Canadian and so on.

And, as we have just been reminded, it doesn’t end there. If you learn English from a Texan you won’t sound much like your friend taking lessons from a Bostonian. British English is an equally broad umbrella, covering not just Scottish, Welsh and Irish accents but countless local variants. Study English under a man from Belfast, and you’ll end up a double outsider in Dublin. The result is a kind of audio-rainbow, apparent in all its glory in the voices of CNN International’s newscasters. Every one of them is fluent, but none sounds quite like anyone else.

What, then, is the aspiring English-language student to do? It used to be thought — and still is by admittedly biased commentators such as Prince Charles — that there is such a thing as a global standard for English, and that it is to be found in the home counties of England.

In the United States, similar claims are made for a kind of educated East Coast accent, shorn of the trademark vowels of New England or the South or the Midwest. But it is not just a matter of accents, noticeable though those are. Conventions of spelling, punctuation and propriety diverge, as well. And yet there is no authority that can pronounce any English-speaking group superior or inferior — to the other or to any other educated speech community. They are all merely different.

That being the case, the student’s path is clear. The goal is to be understood yet inconspicuous. No point being associated with a minority accent, including Prince Charles’ own, when you’re a foreigner already. But as for finding an exponent of standard English, you won’t.

The language is as various as the lands it has penetrated. Just find a teacher you like, and do your best. Then go on vacation in his or her home country.

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