> English doesn't share the same linguistic root. […] Well, if you take out indo-european that is.

It's a bit more complex, English is half a Germanic language but also half French.

But that doesn't explains why “Celtic” is pronounced differently in “Boston Celtics”.

That I presume is just the natural evolution of language, their origins fade away, they become culturally assimilated, and pronounced more in line with the hosts. Nothing new. But interesting nonetheless. I don't have a definitive answer but I presume it was something like that.