If you add Serbian language, I can do the translation. Serbian is similar to Croatian, Bosnian and Montenegrin (in the way Austrian German is similar to Bavarian German, for example), so that should cover those countries too, until a proper translation is done by someone...
I'm confused, which Serbian should I add?
To make things simpler, I suggest you go with Serbian (Latin).
Serbian (Cyrillic) is mostly used in Serbia, so Latin is of greater use to people in Croatia and Bosnia, I guess, while still perfectly understandable by Serbians and Montenegrin. Also, Serbo-Croatian (or Croato-Serbian) is officially no more, so I would avoid it, not to spark political debates. If we use Serbian (Latin) now, everyone in Serbia, Montenegro, Croatia and Bosnia would understand it and you can add Croatian, Montenegrin and Bosnian later, when/if translated.
Personally, I don't mind either way and changing a text from Latin to Cyrillic is literally just one click.
Added Serbian Latin