> This has historically been the philosophy of English linguists
It's not unique to English linguists, it's a tenet of modern linguistics in general. A language is defined by the way people actually speak. If that's influenced by a central organization, fine, but that does not contradict descriptivism at all. Someone studying a language should always study the way the language is spoken by real people, using prescriptivist sources as supplementary sources of information where needed.
I don't disagree, it is certainly not unique to English, and these central institutions do largely have a descriptivist attitude (although the French are known to be rather purist ;) ).
But there is a practical difference: textbooks and dictionaries in English have traditionally come from distributed institutions, which are eminent but none of them claims to be official, whereas for example in Spanish they all originate from or closely follow the standards of the Real Academia.
Sometimes unified standards have been artificially created, like for Basque or Mandarin, and in those cases prescriptivism is more dominant.
It's not unique to English linguists, it's a tenet of modern linguistics in general. A language is defined by the way people actually speak. If that's influenced by a central organization, fine, but that does not contradict descriptivism at all. Someone studying a language should always study the way the language is spoken by real people, using prescriptivist sources as supplementary sources of information where needed.