Thanks for the suggestion about altering "Celtic" to "Breton". I hadn't sufficiently searched the world_names configuration file to see that there was a separate "Celtic" culture along with the national Celtic cultures.
I agree with using some such culture for the province of Cornwall as well. After all, it wasn't until the 18th century that the Cornish language died out (though it is currently enjoying something of a studied renaissance). As for the languages themselves, despite the influence of French on Breton, Welsh and Breton are still mutually intelligible languages. No doubt Cornish would be mutually intelligible as well.
As for Brythonic, the 11th century is really too late for people in Cymry, Cerniu, and the various parts of Bretagne to think of themselves as "Brythonic". And would the people of Cumberland - not Northumberland - and parts of the Duchy of Galloway still be Brythonic in the 11th century? After all, the designation "Cumberland" has the same root as Cambria and Cymry, and the British kingdom of Strathclyde - Stratclut - existed until the 10th century, when it was absorbed into the kingdom of Scotland by marriage.
(This also points out the problem with the province that is named "Northcumberland" and a Duchy of "Cumberland" that is centered in what should be named "Northumberland" or "Northumbria". The map is not alterable, but the country file is. I altered the duchy's name to Northumberland in my own game, and would hope that a subsequent patch does the same.)
All things considered, I think I'll go with Breton, and, given the colonization of Brittany from Cornwall in the 5th and 6th centuries (and later), give that province Breton culture as well (unless I alter "Norse" or some such to "Cornish").
(Is the Norse culture used in any of the provinces, or all they all designated by national Scandinavian cultures?)
As for Scotland, does "Scottish" mean Scots Gaelic culture or the Lallachs "Scots" culture of the Scottish lowlands (essentially a blend of Brythonic and English culture)? Perhaps we should use "Scottish" to designate this lowland culture, and alter "Irish" to "Gaelic" for use in Ireland, the Western Isles, and the Scottish highlands.
