Slightly off-the-wall question, but I can’t think of a better place to ask it:
Does anyone know the Cornish for “Brythonic”?
and/or
Does anyone know the Breton for “Brythonic”?
Diolch yn fawr o flaen llaw!
(I know the Welsh is either Brythonaidd for the adjective, or Brythoneg for the language).