I haven't seen the ad but, if there is a sign which says "Croeso i Gymru" ("Welcome to Wales"), it is because the "i" causes a soft mutation to the "C" of "Cymru" (Wales).
Thanks steve I'm acctually Welsh - but not welsh speaking and I never knew that - shows I must have gone around with my eyes closed when I was little - I live in big old england now so thats my excuse - I'm still facinated with all things welsh though -- thank you
I think i has to do with the word before Cymru being a vowel. like in English a word beginning with a.e.i.o.u will have an an put before it, with welsh it changes the first letter of the word after ie Croeso e Cymru ( welcome to Wales) becomes Croeso e Gymru, i think
I'm a fluent welsh speaker, and I can confirm that the first posted answer is correct. there are a number of different types of mutations in the Welsh language, and the soft mutation rules would mean that the first letter of any word following 'am', 'ar', 'at', 'gan', 'heb', 'i', 'o', 'dan', 'tros', 'trwy', 'wth' or 'hyd' will mutate, as long as it begins with a C, D, or a number of other letters. Welsh is one of the most difficult languages to learn...