\ProvidesPackage{serbianpart}
\renewcommand*{\thepart}{\ifcase\value{part}\or Prvi\or Drugi\or 
 Tre\'{c}i\or \v{C}evrti\or Peti\or \v{S}esti\or Sedmi\or Osmi\or
 Deveti\or Deseti\or Jedanaesti\or Dvanaesti\or Trinaesti\or
 \v{C}etrnaesti\or Petnaesti\or \v{S}esnaesti\or Sedamnaesti\or
 Osamnaesti\or Devetnaesti\or Dvadeseti
\fi\space}