How about the Romanization of the Celtic World?
In our modern era, Celtic languages only (barely) survive in Ireland (Irish), Scotland (Scottish Gaelic), Wales (Welsh), the Isle of Man (Manx) and Cornwall (Cornish) in England, and Brittany (Breton) 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.