I'd argue it stems from the invention of the nation state and the need to build national identity. Specifically in those areas of Germany/France that blended to

I'd argue it stems from the invention of the nation state and the need to build national identity. Specifically in those areas of Germany/France that blended together. Prior to the current school system you didn't get taught a standard language. More specifically, the modern school system is to teach a nation a common shared language.