1

English, by far.

Serbo-Croatian will do for two of the places you mention, and also for Croatia and Montenegro. But it won't get you anywhere in Greece and will only help a little, with recognizing cognates, in Bulgaria.

You should at least learn the Greek and Cyrillic alphabets. After that maybe the language of the country where you plan to spend the most time.

2

Yes, English. You may also find that learning a few small phrases in the partucular language may encourage the local person to try and reply in English. You state that you are planning on visiting the Balkans in the next decade. That gives you ample time as suggested by @#1 to learn either the Greek or Cyrillic alphabets. With that duration you should be able to learn much of the languages too.

3

Another useful language in this region is German, because German tourists outnumber the rest quite substantially, and you will sometimes encounter locals who can speak some German but no English, though this is less common than it was 20 years ago. The exceptions to this are Albania and Romania, where Italian is more useful, because both of these countries, especially Albania, have strong links to Italy.

I don't claim to speak German, but on quite a few occasions just knowing a few basic phrases of tourist German, which is about my limit, have enabled communication when the English wasn't working. The trouble with languages like Serbo-Croat is that, unlike English or German or Spanish, a little bit of it doesn't get you very far at all. Also, the locals are not used to communicating with people who have just a little Serbo-Croat, so showing any facility at all is at risk of getting you a long flow of unsimplified fast native speech. Greek isn't quite so tricky, but it will still take more effort than Spanish/French/German to get much useful facility. Though clearly in each country a bit of vocab to understand menus, and please and thank you, will be useful on occasion.

The Greek and Cyrillic writing systems are closely related, so once you have one you'll discover you have a lot of the other.

4

I had to resort to Italian once in Croatia too (Zadar). Long time ago, though. And German, French, and Arabic in Istanbul, if that's the Balkans.

But I don't think I would recommend studying German or Italian (or French or Arabic) in preparation for a trip to the Balkans.

5

I do not speak Romanian apart from hello, goodbye, please, thank you etc. I have also heard though, that it has a few similarities with both Italian and Latin. Goodbye in Italian arri vederci and La revederii in Romanian. Excuse the spelling.

6

It depends what you mean by studying. Certainly not worth embarking on a course of it. But if you were only proposing to put something like an hour or two into it, you might actually learn a few useful phrases of German such that you could actually employ them. That is about the limit of my German study, but I can deploy useful phrases in that language, simply because of its proximity to English.

Whereas a brief encounter with Serbo-Croat or Greek, you'd only really be at the single word rather than phrase level.