During the decline of Roman power, the mother tongue languages of the imperial core diverged from Latin, and I have the impression that Latin as a "global" lingua franca stagnated. So villages (in today's Italy, France, Spain) that presumably spoke prestige Latin circa 50 AD were speaking various Romance tongues by 1000 AD. But the Latin of philosophy, the church, and Byzantium circa 1000 would have been intelligible to Cicero? I think?
I wonder what that process will look like with global English over the next millennium, and I wonder what difference there is with the history of Chinese as an imperial language. It tickles my fancy to imagine the divergences of patois in the middle of north america.
Inspired by https://polyglot.city/@Stoori/106386418776867251