As you can see in the screenshot from my question - it is a new version (and due to that I can select/pick/add new languages). In case of already released apps the selector is restricted to only the chosen/predefined languages (no plus sign available): Therefore I believe that the Serbian language is just not supported (?), and as far as my research goes, the default language for Serbian users in App Store is English. So I'll leave it (English) as the only defined option for my app and put Serbian content in it... not a pretty solution, but don't see a better option for now.

I hope it would provide information whether or not the Serbian language is supported, this would be enough.