[enriquto]

This is not exactly true. There are some fundamental differences between spanish dialects. For example, "yo", "calle", are pronounced in an unrecognizable way between Argentina and Spain. But I agree that the mapping from written to spoken spanish is mostly deterministic (like that for French, Italian, and other romance languages). The mapping from spoken to written, of course, is not deterministic at all, which you can verify by looking at how illiterate spanish-speakers try to write.