>English doesn't have the equivalent of Académie française, and even its edicts are well-regarded recommendations, not definitions.
Even if it did, I suspect either the US or the UK would reject the country that wrote it.
>They with a singular antecedent has remained in common use for centuries in spite of its proscription by traditional grammarians.
I was being a traditional grammarian. "John has a flower. Their flower smells good," sounds as incorrect to me as, "John ain't got no flowers." For me, the more important the point I'm trying to make the more grammatically correct I try to be.
Of course the spoken word is completely different. I think a message can get a lot of additional value misusing the language.
Maybe I should just realize, "ain't nobody got time for that."