We have very little surviving evidence of Pictish - mostly just place names and personal names. Based on these, Pictish appears to be a Brittonic language, which is a branch of the P-Celtic languages, which are, in turn, a major branch of Celtic languages.

The surviving Brittonic languages are Welsh, Cornish, and Breton. Welsh appears to be the closest surviving relative of Pictish, although Welsh, Cornish, and Breton are likely to be closer to each other than Pictish was to Welsh.

This isn't universally agreed, and not that long ago was widely disagreed with (see below for more on this, and Rhys (2015) below).

Lots of talk of their culture, but not of their language!

This is probably a result of 2 things. First, the limited evidence for their language. Second, we don't know much detail about their language - we can say "somewhat like Welsh", or perhaps discuss the controversy about the classification of Pictish, but then much of what can be said outside specialist works on the topic has been said.

Which brings me to suggest a specialist work on the topic if you want more:

• Guto Rhys, Approaching the Pictish language: historiography, early evidence and the question of Pritenic, PhD thesis, University of Glasgow, 2015: http://theses.gla.ac.uk/6285/7/2015RhysPhD.pdf

See page 118 for a discussion of the consensus that Pictish is a Brittonic language. While this is an old idea, it is only quite recently that it has been possible to describe this as a consensus - much older work was firmly in the camp that Pictish was non-Indo-European (note that Celtic languages are Indo-European). For example, John Rhys, in

• Rhys, J (1892), "The inscriptions and language of the Northern Picts", Proceedings of the Society of Antiquaries of Scotland, 26: 263–351: https://archaeologydataservice.ac.uk/archiveDS/archiveDownload?t=arch-352-1/dissemination/pdf/vol_026/26_263_351.pdf

• Rhys, J (1898), "A revised account of the inscriptions of the Northern Picts", Proceedings of the Society of Antiquaries of Scotland, 32: 324–398: https://archaeologydataservice.ac.uk/archiveDS/archiveDownload?t=arch-352-1/dissemination/pdf/vol_032/32_324_398.pdf

argued that Pictish was non-Indo-European, related to Basque in the first of these papers, and then abandoning that but still maintaining that it was non-Indo-European in the second. (Basque is a famously mysterious language, one of the few non-Indo-European and non-Uralic languages in Europe. Possibly it came to Iberia with copper metallurgy from the Balkans.)