The color of one's hair is determined by DNA (deoxyribonucleic acid) or genes. The language one speaks is determined by cultural movements, which is quite another thing. Scientists have determined that the people who inhabited Europe came in a number of waves, beginning about 45 thousand years (kya). The first ones appeared in southern Russia, probably having come through the Near East first, and from Africa before that. Originally all had dark hair, in fact, it was probably black like modern Africans and southern Indians and Australians (adults, that is). As people moved north, where there were still glaciers and ice caps, they needed to be able to absorb more sunlight through their skins, so their DNA changed and they became paler, through mutations. As this happened, probably originally in Central Asia, as the DNA studies show, some eventually also developed light-colored hair. The studies indicate that pale hair originated in only one place, and that originally it was fairly rare. People spread out from somewhere in northern Europe with this pale hair to the various places where they are now found. The spread of people speaking various languages is more complicated and would have to take up a lot more space than I have here. If you are really interested, look up Indo-European languages and follow that language family to its roots, then trace the Celtic family within it, then the specific groups within that. Wikipedia is a good place to start.