It's the family-size, ten-gallon pail of worms you've opened up.

Language changes over geography, across social strata, and through time. Pronouncing on "correct" forms has to be accompanied by cautions about the combination of these considerations. Language is also pure convention -- what's "right" is not what's historical, but rather, the people who are in a position to impose their will get to rewrite history (ex: the stigmatized usage "I axed him a question" is historically correct, but the population who initiated the error [all language change begins as error] trumped the conservative speakers).

K vs S seems an unlikely alternation, but it's the basic dividing line among the Indo-European languages. In very general terms, the eastern IE languages rendered the IE *kmtom (100) something like the Sanskrit "satem," the western languages producing "centum." (an unvoiced velar stop, or a "hard c"; in the Germanic languages, of which English is one, the stop drops further back in the throat and turns into a fricative -- "H" for "hundred"). So Germanic words beginning with "H" are ultimately "K" sounds (and correspond to, say, Russian words beginning with "S": heart/sirtse).

Another alternation involves voicing of the K to G (buzzing of the vocal folds). So we get the related terms "Kelt" and "Gael." For reasons nobody has explained satisfactorily, there's a translinguistic tendency for "G" to morph into "W," so that Gaels are ultimately connected with Wales (and Galicia, Galatia, Wallachia, Gaul -- those folks were ALL over).

All of this seems less bizarre if we try to put our heads back into the pre-literate days when nobody had any notion of correct spelling. It startles many to realize that there's no "t" sound in "battle," or that the vowels of present day English phonology (not spelling -- a different matter) include M,N, L, and R.

So "Kelt" and "Selt" have historical warrant (so does "chelt"). But people get all proprietary about these things, and invest them with political meaning. Down my way, you can start a bar-brawl just by insisting on "Missouree" or "Missourah".

Adam