Pictish is a dead language. It was probably yet another Celtic language, but we don’t have enough information to rule out anyone’s pet theory. It is a remnant language that lives on in a few possible given names, place names and that’s about it.
This is at least the second time I’ve seen a movie with Picts that weren’t speaking Pictish. First was King Arthur- the Picts spoke English. Now in the The Eagle, they Pict speak Gaelic, which is good for Gaelic to get some screen time, but this was also an opportunity for a director to hire a conlang creator.
What options are there?
Embrace Gaelic, Scots
Gaelic and Scots are rather endangered languages, so anything that can create a some use for them would be a good thing. Maybe we can do some good for a living language by using it, instead of the impossible task of reviving a nearly, completely dead language.
On the down side, there isn’t much in the way of a technical or design challenge here for the conlanger.
Create a Pict inspired Conlang
We know about the Pict’s physical culture, a handful of names that were possibly Pictish. From that we could with meticulous effort maybe construct phonetic inventory a plausible phonotactics. Next, we’d have to pick a language family. Since the Pict’s physical culture was almost identical to the surrounding people, it follow they may have spoken a language in the same family– so something Indoeuropean, and of the options, probably something Celtic. If it wasn’t, then one would have to either create an isolate– without any recognizable link to any living language–or if you like Ruhlens and Greenburgs work, something that looks like it could be in the Indo-Euralic family. (Or a minor variation on the idea, the Nostratic superfamily)
So a conlang Pict would be a known Celtic language with enough changes to make it sound like a Celtic language without really being mutually intelligible.
As a movie language, it would be constrained by how pronounceable it was for the actors and if it sounded pleasant or appropriate to the audience.
Also, unlike some remnant languages, like Virginian Algonquin, there isn’t any pre-existing corpus for which one needs to maintain compatibility. Meticulously extracting all the information about a language from a series of word lists and a few sentences is a huge task that takes scholars years. Movie conlangs need to be created on time and on budget, so there isn’t room for decades of decipherment. In Picts case, the conlang creater is very rapidly at the point where they have to choice but to make things up, and will not have the opportunity to be tempted to write a conlang that is a laborious reconstruction in the same way that PIE has been reconstructed.