There is heated debate whether Pictish as a language was Brythonic or Gaelic (or indeed otherwise) in origin. The liklihood is, however, it was a Celtic language. Discoveries of Pictish stones in Scotland do not give rise to fully conclusive evidence to aid us in working out the language’s definite origins.
There is no written evidence for the language of the Picts. It is, in my view, more than likely that several dialects were spoken due to the landscape and terrain being so vast and difficult to inhabit.
Picts seemed to be very fond of carving shapes, symbols and pictures into rocks. Carvings were usually made on the flat, front-faceing sides of large standing stones – unlike Ogham which used the sides of stones on which to carve.
As for understanding these stones, our only hope would lie in the discovery of a ‘Rosetta Stone’-like example comprising of both Pictish symboligy and Ogham. Aside from this, the only other possiblity would be in parts of some place-names.
I have, for what it’s worth, included some links I find interesting in Pictish below: