Even if you want to ignore the fact that when a Celtic word is Latinized "c" is always pronounced "k" (no one says Son-so-var, instead of Kon-ko-var [written as Conchobhar], or Sue-sue-lin, as opposed to Ku-Ku-lin (or K-K-lin) (written as Cu Chulainn)), as the word is Greek, the hard k should be kept anyway - nouns are always (or at least should be) transliterated.

In my experience, the word Celtic, when pronounced Seltic, has nothing to do with the Celts anyway.

So, one should prononce the name "Keltia" not "Seltia".