Usage. Correct English is determined by the usage of English
speakers.
Or more precisely, by the dominant usage in the language-use context.
Usage varies, of course. Some forms predominate, due to various
factors such as popularity, use by the politically powerful classes,
presence in various media, and so forth. But they predominate only
in specific contexts; in others, regional or other variations may be
more influential in determining what a given speaker or author and
audience will consider "correct".
Of course, many usages get codified, and many users like to point to
such codices as authorities, but they're no more authoritative than
users grant. They have no special status.
As a native English speaker, I admit no other authority on the matter.
A reasonable position, and the right one in my view, though of course
any English speaker can choose to defer to some language "authority".
What they can't do is demonstrate that it applies, or should apply,
to all English speakers.
Well, there are certainly people who maintain such lists, such as
the authors of English grammar and usage primers. Whatever authority
they have comes solely from the decisions made to publish and purchase
those books.
That said, I imagine I'd consider ISO 3166 a standard for English
spelling of country names, though I'd want to read it first before I
committed to that position. And clearly it is "official" in a useful
sense. However, I wouldn't call it an "official English standard";
I'd call it an "official standard for spellings (of country names) in
English". That may seem like a fine distinction, but I think it's
significant, as it implies that its office pertains to certain
spellings rather than to English per se.