The Celts originally occupied the Iberian peninsula, Gaul, Germania, Ireland, and Britain. What we know of the Celts comes primarily from the Romans, since the Celts didn’t have a written language. What little bits of writing remain from the Celts was near the early Roman Empire, and in vulgar Latin.

Of course, the Romans ultimately conquered a lot of those areas, into Britain up to about Hadrian’s Wall. I believe they semi-conquered Cornwall and Wales, to the extent that they “ruled” those areas, but still left them generally to their own devices. Only Wales really retained its relative independence, as is evidenced by the fact that Welsh still exists, while Cornish is barely holding on after having become extinct and then revived.

The reason I compared “pagan” to “hillbilly” and “redneck” is because plenty of people wear those names with pride, too, even while many other people continue to use them in derogatory ways.