I find myself, before a Scottish audience, in a position of phonetic ambiguity. Do I say Kelt and Keltic, or Selt and Seltic? As an Englishman, I use the former pronounciation from habit, though aware that Scottish usage favours the soft ‘c’, as does the O.E.D., giving ‘Keltic’ only as a secondary alternative.
from Stuart Piggot, Celts, Saxons and the Early Antiquarians, The O’Donnell Lecture 1966, Edinburgh University Press, 1967, p.4