In VLC, while using any font that supports several versions of these letters (which depend on the system language and/or application language—such fonts include Segoe UI, Calibri & Noto), they are always displayed the Russian way, even with the system and application language set to Serbian.
Here’s an example. Say the user’s VLC client here is set to Serbian; letters in cyan represent the way glyphs should appear while white letters show them how they do appear.
Something like this is easy to change in, say, HTML: you can use the <html lang= parameter and change its value to sr-rs, at which point all Cyrillic italic glyphs on the page will appear as Serbian. Seeing that VLC already supports various character encodings for subtitles, I’m guessing something similar can be implemented in order to change character glyphs between different languages?
More relevant details and examples can be found here:
https://en.wikipedia.org/wiki/Serbian_Cyrillic_alphabet?oldformat=true#Differences_from_other_Cyrillic_alphabets
