Catalogue of the  vital records available in the Zadar archive - DAZD:

http://arhinet.arhiv.hr/_Pages/PdfFile.aspx?Id=591

Neither Old Church Slavonic nor Serbian language are identified as the language any of these records are written in  - even for those that are marked as *pravoslavni* (Orthodox).

You will need to officially ask specifics in many cases - if the records are original parish records or the civil copy, if the civil copies have all residents in them or only of one congregation,  if the church  records not identified by any marker (language, script)  are maybe Orhtodox or  Greek-Eastern...  - compare the DAZD catalogue  listing with the listing of the LDS microfilms for the Serbian Orthodox  parish of Ervenik and you will see  that only three of the  fifteen records LDS microfilmed are identified as being in cyrillic script, and NONE are identified in the DAZD catalogue as *pravoslavni*.:

https://www.familysearch.org/search/catalog/show?uri=http%3A%2F%2Fcatalog-search-api%3A8080%2Fwww-catalogapi-webservice%2Fitem%2F674130

Unfortunately, this particular archive is not   * user friendly* if you are corresponding with them about researching Serbian ancestors, and probably the only way to receive accurate information and research their holdings is to send someone to the archive to look at the records for you and identify the ones you need.