In certain parts of Spain, 'C's and 'Z's are often pronounced as 'TH's. Whereas, I might be inclined to express my thanks with "Grassy-ass", a local will say "Gra-thi-as" or "Gra-thi-a." Similarly, the word "plaza" is spoken as "pla-tha."
Our bus to Portimao, Portugal, leaves in a couple hours. I'm told the pronunciation of Portuguese can be tricky for foreigners; the vowel sounds are often muted, or nasalised. Our phrase book describes one tourists impression of the language, as sounding like "a drunken Frenchman trying to speak Spanish."