That would be redundant since the Polyglossia package already takes care of selecting alphabet & language, thus also the relevant localized glyphs if needed.
\usepackage{Polyglossia}
\setmainlanguage{Serbian}
However I tried to add:
\defaultfontfeatures{Language=Serbian, Script=Cyrillic,
Mapping=tex-text}
But it did not change the output.
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