There is an announcement, that 20 new countries will be supported by the App Store... and that developers need to update their PLA till the 10th of April. But there is no word about when will support be activated.
If Serbia is in the list of newly supported countries, I would bet it will rapidly be activated. But that's just a personal bet.
In the meantime, you can prepare the serbian version of your app, if not yet done.
Yes, Serbia is on the list but I tough that maybe the date is announced somewhere.
Anyway just like you said I'll do what is up to me.