Re: Is this just an English thing ?
Since the 15th century at least, they and you are both singular and plural, depending on context. Probably the 14th too.
The PM came onto the stage. They said "ptaugh fwah!". The crowd cheered, even though they had no idea what it meant.
Teresa May stood at the sidelines. "Oi, You!", she exclaimed. "Come over here and say that"
Perhaps you should read a few books?