"The letters s and z are used interchangeably in the Czech language...."
?! Not so. The only thing I can think of that you might be referring to is that, in pronunciation, most consonants come in pairs, one "voiced" and the other "unvoiced," and the position in which a consonant occurs within a word can cause a "voiced" consonant (e.g., "z" or "b") to be spoken like its "unvoiced" counterpart (e.g., "S" or "p"). But this is a general phenomenon of speech, not confined to Czech. The same thing occurs in English (e.g., "dishes" is pronounced "dishiz" but "plates" is pronounced "plats"), but we learn to ignore it. In some cases in Czech, the true pronunciation gets reflected in the accepted spelling of a word. But usually not; for example, "dub" (=oak tree) is correctly pronounced "doop."