The Roman Empire imposed Latin across most of Western Europe between roughly 200 BCE and 200 CE. Latin became the language of administration, law, the church, and education from Britain to North Africa. But the Latin spoken by everyday Roman soldiers, traders, and farmers was not the polished language of Cicero. It was a colloquial register, called Vulgar Latin, full of slang and shortcuts.

When the Western Empire collapsed in the 5th century, the political glue holding Vulgar Latin together dissolved. Local communities continued speaking it, but each region's version drifted independently. Spanish farmers, Frankish soldiers, Roman peasants, and Iberian fishermen kept evolving their speech without coordinating.

By 800 CE, the differences were big enough to be a problem. Charlemagne's clerics complained that ordinary people could no longer understand Church Latin during sermons. The Council of Tours in 813 ordered priests to preach in lingua romana rustica — the rustic Roman tongue — because nobody understood the educated form anymore.

By 1000 CE, the regional dialects were no longer Latin at all. They had become French in Gaul, Spanish on the Iberian Peninsula, Portuguese on its western coast, Italian on the peninsula itself, and Romanian in a strange isolated pocket east of the Carpathians where Roman colonists had stuck. Each was now mutually unintelligible with the others.

Latin itself, the formal classical language, was preserved separately by the Catholic Church and by scholars. It became the universal language of European learning until about 1700. It still functions as the official language of the Vatican.

Most languages don't die. They speciate. Latin is the textbook example.