Most serious linguists now think Pictish was a Celtic language rather closer to Irish than Breton/Welsh/Gaulish. Though there isn’t a lot to go on. Here is the entire corpus of Pictish inscriptions:
Location Inscription [tentative translation]
Abernethy qmi
Aboyne nehhtvrobbaccennevv maqqotalluorrh [Nechtan … son of Talorc]
Altyre ammaqqtallv lv bahhrrassudds [..son of Talorc …]
Auquhollie vuunon itedovob b
Birsay 1 (m)onnorranrr
2 bqi a b
Brandsbutt irataddoarens
Bressay crroscc:nahhtvvddadds:dattr:ann
bennises:meqqddrroann [… Nechtan … son of Drostan]
Brodie A von…ecco..
B rginngchqodtosombs
C eddarrnonn… tti… gng.. [Pidarnoin …]
Buckquoy (e)tmiqavsallc
Burrian idbmirrhannurractkevvcerroccs
Cunningsburgh 1 iru
2 ..ehteconmors
…dov …ddrs
3 etteca…
..v:dattua
…rtt..
Dunadd: hcsd.t..v.nh.t
l…vqrrhmdnhq
Golspie: allhhallorreddmaqqnuuvvhrre.rr [… son of …]
Gurness Broch ineittemen mats
Inchyra A ettlietrenoiddors
..uhtuoaged…
B inehhetestieq…inne
Keiss Bay nehtetri
Latheron duv nodnnatmaqqnahhto… [… son of Nechtan]
Logie Elphonstone caltchu
Lunnasting ettecuhetts:ahehhttannn:hccvvevv:nehhtons [… Nechtan]
Newton iddarqnnnvorrenn iku(a) iosie
North Uist m..quntenac..t
St Ninians (…)besmeqqnanammovvez [… son of …]
Scoonie eddarrnonn [Pidarnoin]
Weeting 1 ulucuvute
2 gedevem…dos
Whiteness …vndar
(Other theories, shading into crankiness, relate Pictish to Basque or Lappish. I prefer to believe that the inscriptions were left behind by proto-Polynesians as a waste dump for all the consonants they wouldn’t be needing any more when they got to the Pacific).