I have checked all letters on Serbian keyboard and all letters work correctly for Serbian standard keyboard, except letter ж, so I provided a fix for that. Capital letter Ж was working as expected.
@Milicevic01 says in the description: "when i type letter Z on keybord i should get cyrillic З, but i got S".
That's also expected. Serbian keyboard differs from English keyboard and letters Z and Y switch places. You probably have (or had at the time of writing this) English keyboard, and think that by pressing Z you would actually get Z, but you get Y. When typing in Cyrillic, pressing Z on English keyboard gives S and pressing Y gives З.