So, I've been reading a bit about the Picts recently.
I'm not sure there's a consensus on exactly who they were or what type of language they spoke.
Even their name - Picts - has more than one explanation of where it came from. I've read some scholars believe it comes from Latin pictus (painted), referring to their supposed habit of painting themselves and/or tattooing themselves. Other scholars believe that the name Pict comes from another source.
I've read several passages from Greek & Latin historians that refer to the Picts. They are usually described as red-haired.
So, people with tattoos and red hair ... sounds like Glasgow on a Friday night.
The Picts were a confederacy of tribes located in central & northern Scotland. They had their own language(s) and culture. Before the Viking Age, Orkney and Shetland were part of the Pictish realm, I think.
What was the Pictish language like? Well, I don't know.
And scholars aren't 100% sure, either. Some believe Pictish was a Brythonic language.
Brythonic languages are a group of Celtic languages, called P-Celtic because IndoEuropean k becomes p in this language group. Compare the Welsh numerals 4 & 5, pedwar and pump, to the corresponding Latin quattuor and quinque.
The Brythonic languages (also called Brittonic) were spoken all across what became England and southern Scotland - these languages were spoken by ancient Britons and eventually evolved into Welsh, Cumbric, Cornish and Breton.
Other scholars believe Pictish to be a separate branch within Celtic or a different IndoEuropean language altogether.
Here's were it starts to get confusing ... some time in the 5th century CE, Gaels from Ireland came to Scotland. It would've been a short journey - apparently you can stand on a beach in northern Ireland and see Scotland on a clear day. These Gaels established a kingdom called Dal Riata which straddled parts of northern Ireland and south west Scotland. The Gaels, naturally, spoke Gaelic.
The Gaelic languages comprise another branch of the Celtic family, and are called Q-Celtic because IndoEuropean k became q (later c) in these languages. (Compare/contrast the numerals for 4 & 5 in Scottish Gaelic, ceithir, coig and in Welsh pedwar, pump.)
In late antiquity/early middle ages, the Pictish kingdom either merged with Dal Riata or was absorbed into it, becoming the kingdom of Alba. A process of Gaelicisation happened, the Picts apparently stopped using their own language and switched to Gaelic. Pictish kings adopted Gaelic names. For instance I read about a Pictish king called Angus mac Fergusa. In Gaelic the noun mac means 'son'; combined with another name it means 'son of'. I did read that Angus mac Fergusa had a Pictish equivalent which I can't remember off the top of my head, except that mac was replaced with map before the second name. So map must be Pictish for 'son of'. I find this interesting because the Welsh word for 'son' is mab. So, was Pictish a Brythonic language, like Welsh? Well, maybe.
Also worth considering is the fact that Welsh wasn't confined to Wales. Historically, Welsh, or a similar language, was spoken in parts of north west England and southern Scotland. The name Cumbria is very similar to the Welsh name for Wales - Cymru (pronounced 'cum-ree') Also, Wallace is a fairly common name for lowland Scots. It means 'Welsh'. The Welsh called these parts of north west England & southern Scotland Yr Hen Ogledd (the Old North).
So, who were the Picts?
I don't know for sure.
Were they a single group of similar tribes, or were they a group of completely unrelated tribes speaking unrelated languages?
They might have been ancient British tribes, similar to the Welsh and Cornish, or they could have been completely different.