Andrew: I call it "Yugoslavian language", because Bosnian, Serbian, Croatian, Slovenian, Macedonian, Montenegran, are all pretty much the same, and picking one name is picking a side. As they say, learn Serbian, you know 6 languages.
Indeed in Republika Srbska, and Serbia, Cyrillic is the script. In Serbia, they use Latin and Cyrillic. In Republika Srbska, it's all Cyrillic. I can read it, but often don't know the meaning of the words. And, yes, often there is a little English, but it's much less likely to have a little English there than in, say, Bretagne or Catalonia. We spent time in Serbia going to small villages looking for family graves, and there were a number in which no English was spoken. I could get by sometimes with a little German, and we had a Serbian guide from the Beograd TI. i don't usually go with guides, but it was a good idea in Serbia, and I think depending on your interest, possibly in Bosnia as well.
We had a driver take us by car from Beograd to Sarajevo - about 300 €, which was comparable to airfare, but he dropped us right at the hotel. We also had a driver from Sarajevo to Mostar. Since buses had stopped from Mostar to Dubrovnik, he drove us to Dubrovnik, but via Kotor. Again, not hugely expensive.
Good point about the mines. Not a place to go for a stroll in the woods.