This might be a very nice incentive as it would bring more cooperation within the region in this community, so I can't say that uniting under a same board is a bad idea. However, the reason for a need of a local board is not just for a language, but for the topic of discussion.

In Serbian topic people discuss a lot about Bitcoin related topics that are specific to the country.
As Serbia is not part of the EU, it has a huge difficulty in buying and selling Bitcoin and that is being discussed a lot. It is kinda isolated economically.
Mining is probably discussed quite differently in Serbian topic as well, since I think that electricity is substantially cheaper in Serbia.
Laws, regulations and it's enforcement are also very different and the economic capability (of acquiring hardware for example) is very different as well.
All of these very different situations bring very different topics of discussions that are irrelevant for all the other boards.

I am afraid that Serbian users would have to scroll just as much through this combined board to find relevant topics, as they already have to do in their current local topic. The issue would just be brought to the Croatian users as well, since they would have to scroll through topics that are irrelevant to them as Serbian users all switch to this board.

So it is a very nice thought that maybe could be implemented in order to bring people together, but it kinda defeats the purpose of local boards in the first place.