When I was a post-graduate student in London 20+ years ago, the preferred pronunciation in Early Medieval Archaelogy & Literature circles was Kelt (with a hard C) when referring to people and languages, and Selt (with a soft C) when referring to stone axes and American sports teams (a very rare occurrance). But as has been pointed out, "correct" pronunciation often depends on time, place and company. A street in New York and a city in Texas share the same name, Houston, but the pronunciation is wildly different and visitors are expected to conform to the local norm or face ridicule.