Conveniently enough, France does have a sort of equivalent to Wales, a rainy, mountainous, coastal region with its own Celtic language and a lot of stone circles: Brittany. Breton does not make much use of the combination ll, but it does have a few characteristics that appear equally exotic to French speakers - in particular, the combination c'h (transcribing the velar fricative /x/) and the frequent use of the letter k (for /k/, reasonably enough). So Kreskenn Kelenn (one guess as to the name's meaning in Breton) talks like this:
Je vois un homme ki tient une hac'he de jet !
So in this case, it works out quite well - though I imagine the joke is lost on readers from, say, Quebec.
I gather that Soul Music has been translated into quite a few languages, but I don't think Arabic is one of them. What on earth would a translator do in this case? It would be kind of tempting to go for equating Celts with Berbers - there are a few stone circles in North Africa - and have Imp substitute ث ذ for ت د. But I don't think any Arab reader east of Algeria would get the allusion, and I doubt that the Middle East contains any ethnic group that can be satisfactorily thought of as playing the role for the Arabs that the Welsh do for the English. Then again, if I were an Arabic translator asked to take on Soul Music, I would give up immediately - any of the few Arabic speakers capable of getting enough of the rock music history allusions to be entertained by the book would be more comfortable reading it in English or French anyway. But that objection is not insuperable: after all, The Wasteland and Finnegan's Wake have been translated into Arabic (for some reason). Perhaps some day a genius will come along sufficiently reckless to give it a try...