The consensus amongst linguists is that the Picts spoke a northern variety of Brythonic, which lacked the Latin influence that the Brythonic language further south received after centuries of Roman rule, this is consistent with personal names as well as placenames, the numerous Aber, and Strath placenames for example are all very Welsh.
Cumbric, another Brythonic language, spoken in Southern Scotland and Northern England also differed to some extent from the Brythonic dialects spoken further south, Just as Cornish, Breton and Welsh differ, yet no one would deny that they were all Britons. While Pictish culture and language had diverged to some extent, such as the Pictish stones and possible matrilineal inheritance, they were still Brythonic Celts, speaking a Brythonic language and were referred to as Britons by the Romans prior to the the 3rd century, Pict was a term that they didn't even call themselves, they possibly called themselves Albidosi, they were called Cruithni in Gaelic and Prydyn in old Welsh, both terms mean Briton.
This is a good indepth read about the Pictish language.
https://theses.gla.ac.uk/6285/7/2015RhysPhD.pdf
Bookmarks