[jerf]

Prior to general travel for everyone being affordable, and broadcast media like television that can go everywhere, languages were affected by the same forces everywhere. So you'd get that effect pretty much everywhere in the world.

Even a lot of things that we think of as "the" version of a language are often effectively a particular dialect out of a complicated tapestry of local dialects being something that "everybody" has to learn because it is the language spoken by your rulers. It happened to "win" because the people speaking that dialect also won the local military conflicts and became the language of the court.

[dhosek]

Indeed, the French taught in schools is Parisian French, but French as its spoken, e.g., in the south sounds noticeably different.

[Bayart]

Parisian French isn't the same as Standard (Court) French, and it sound different in the South because It's only been the majority language for a century. It's super-imposed on top of another language's phonology. It's not a dialectal continuum thing.

[williamdclt]

Not wrong, but note that the difference is much less than language differences between different English speakers in England, even at short geographical distances.