Serbian with Latin script uses in MediaWiki currently the language code sr-el. The module jquery.ui.datepicker uses the language code sr-sr for Serbian with Latin script.
https://translatewiki.net/wiki/Special:TranslationStats?uselang=sr-el shows Serbian Latin with English datepicker:
https://translatewiki.net/wiki/Special:TranslationStats?uselang=sr-sr shows English with Serbian Latin datepicker:
Expected result: Language code sr-el should show Serbian Latin with Serbian Latin datepicker.
For changing sr-el to the BCP 47 conform language code sr-Latn see T117845: Rename the language codes sr-ec and sr-el to the BCP 47 conform codes sr-Cyrl and sr-Latn.