We all speak English... you and thou and yinz and y'all are all English. Each usage as "correct" as the other, when used in its environment.

Prescriptive Grammar went out of fashion many years ago... Jonathan Swift's 1712 position seems silly today:

"Some method should be thought on for ascertaining and fixing our language forever... I see no absolute necessity why any language should be perpetually changing."

Samuel Johnson saw the absurdity of the above, writing in 1755:

"Sounds are too volatile and subtile for legal restraints; to enchain syllables and to lash at the wind are equally the undertakings of pride, unwilling to measure its desires by its strength."

The use of "you" in the singular is a modern simplification, or laziness if you will, which made the language less clear and precise.