Keltic or Seltic frost?

it actually has nothing to do with the actual Gaelic pronuncation as far as I know, because the word is not derived from Gaelic. In talking of celts, it can be used interchangably and still be proper english, but since things like the band or in the sports team, the pronunciation is a particular way since it's an proper name (so it will always be the Seltics and Keltic Frost)
 
I've always pronounced it Celtic with a hard C. I've seen it go either way though. I think it might be an American thing to call the Seltic Frost, though I call them Keltic.