Well, the Welsh language has a Y and the C is always a "k" sound -- most notably in the Welsh word for "Wales", which is "Cymru" (pronounced: "come-ree").
But as for English, you won't have much luck convincing anyone. The best you can say is that in words derived from Greek, the "c" actually represents a "k" sound. Our words "cinema" and "ceramic" ought to be pronounced as if spelled with "k".
But that argument doesn't hold much water for words not of Greek or Celtic origin.