This is what Ive always heard. I thought it was generally accepted they spoke a Brythonic tounge similar in some ways to Welsh.

there is proof they spoke a proto-Celtic language before the mass bronze age migration by the Celts. but this doesnt mean theyre Celts. i would argue it proves the opposite: that the “Celtic” identity originated from that bronze age migration to differentiate themselves from the Picts. even if they share common ancestors only a few centuries prior to that, the Picts are the oldest group we have evidence of existing on terf island.

the fact that they spoke a “proto-Celtic” language only shows that language developed when the Picts’ ancestors lived in mainland europe and there are similarities from the language the Celts used due to common ancestry

or maybe their language was influenced by the Celts after both groups lived on the island