Wales came from the old English 'wealas' - a word meaning 'aliens or foreigners' - a bit rich really! I have heard from another source that Welsch in Swiss German refers to the French-speaking Swiss.

Welsh Cymru/Cymraeg (welsh language)/Cymréig (welsh people) preserves the hard 'c' sound, but has nothing to do with the word keltoi and its derivatives being akin to Cambria (Latin) c.f. Cornish "Kembri" and Breton "Kambre", both meaning Wales.