Fixed and updated Serbian translations (both writing systems) (#2144)

Fixed and updated Serbian translations (both writing systems) (#2144)

* Update Serbian (both writing systems)

Fixed a stray Cyrillic word in the Latin text.
Translated the untranslated strings in both texts.
Changed a word in both texts.
Added part of a string that was missing in the Cyrillic version.
Fixed spelling mistakes in the Cyrillic text.

* Update languages.json

Added myself to the credits of the Cyrillic text too, as I forgot to do so earlier.