France, however, had spoken versions of Latin since before the time of Christ; just the Germanii and everyone north (Scandinavia) didn’t make the switch. On the British Isles that Old German / Old English was common everywhere except Wales, Ireland and Scotland, which the Germanii had dislocated but not dislodged.
As of the Norman Conquest, Old French became the language of the powerful, and Old English began to absorb it. Three hundred years later Chaucer was in high gear (1343 – 1400) and his dialect of Middle English is the one you read in College when you study him. Truth in advertising, his ws one of several hundred little local sub-dialects. His, we saved in written form.
Another two hundred years and we get Shakespeare and the King James Version of the bible, both of which acted like enormous sea anchors. They kept the language relatively stable.
SO – the Norman invasion, Chaucer, Shakespeare, King James—all we need to do is invent a time machine and bring back a thousand copies of the Sun (London) and the Times (New York) from some arbitrary date, same day each year through 3019, and we will know.