How about the Romanization of the Celtic World?
Yes, all of that land was formerly controlled by the Celts. Even Southwest Poland was Celtic. There is an endangered language spoken there called Silesian that has at its very base a Celtic layer which is the oldest layer of this Slavic language. The French language was Celtic Gaulish, the influence of which can still be seen in the odd French phonology. I do not think there is much Celtic left in the Iberian languages, but I could be wrong on that. Surely there is little or no Celtic left in Turkish. One wonders about Celtic traces in Dutch, German and the rest of Slavic.
In our modern era, Celtic languages only (barely) survive in Ireland, Scotland, Wales, the Isle of Man and Cornwall in England, and Brittany in France. In Eastern Europe, Celts were supplanted by Germanic, Iranian and Slavic tribes. In France, Iberia and the Balkans, the Celts were assimilated to the Roman Empire.
It is not particularly difficult to convert a native elite to the language of a conqueror, but converting an entire population to a new language in a short period of time is quite a feat. The Romans did this mostly by showing the superiority of the Latin language and convincing the natives to give up their Celtic words.
In fact, the Romanization of Dacia where the original Celtic speaking people were completely converted to Latin which then turned into Romanian is cited by Wikipedia as one of the worst cultural genocides ever.
Of course there are many other examples of cultural genocide, some of them ongoing.
