Celtic vs. Celtic

Can someone tell me why sometimes it is pronounced with a hard C and sometimes with a soft C?
I used to think it was just a US thing of mispronouncing it (specifically the Boston Celtics), but I've heard it pronounced both ways over here as well, and don't know why.

Or is the whole boadicea thing again? No one can agree on hos it was originally pronounced?
(I, for one, prefer BOO-dikka).

Celtic isn't a language is it? It is a group of languages though. The native Irish language is properly referred to as Irish (as opposed to Gaelic) though I think the Scot's language is referred to as Scottish Gaelic. Don't we have a resident expert on that? It also includes Welsh, Cornish and Breton if I'm not mistaken.

I think Jemima is right. The basketball is the only Seltic. I haven't heard it used here. Do you remember what the context was you heard it in?

Celtic Q (Goidelic): Irish, Scottish Gaelic, Manx
Celtic P (Brythonic): Welsh, Breton, Cornish

I call Irish Gaelic "Irish" or Gàidhlig na h-Éirinn.  I never call it Scottish Gaelic unless I need to draw attention to the fact that it is Scottish.  Even in the language itself, I always say Gàidhlig rather than Gàidhlig na h-Alba and this has been my experience with native speakers as well.

The only time I've ever heard Seltic has been in reference to the football or basketball team.

So why do the Sports teams use a soft C? If that's the only context in which it is used? Seems odd...

Cuz it's a bunch of sports fans?

(just kidding!)