hrvatski/srpski/bosanski jezik
Greetings to everyone.
I'm a Croatian-Russian teenager, and would like to say few words about my primary mother tongue.
First, about the differences. Croatian, Serbian and Bosnian are NOT the same language, to start with. Somebody mentioned they separated because of political reasons, but friends, keep in mind that they formally united for politic reasons as well.
However, they are INTELLIGIBLE to that level that you are free to say that you understand all 3 by knowing only 1. But, friends, let us not deceive ourselves. It is dead easy for me to read Serbian and Bosnian (though I come across some Turkish influence in Bosnian), I read it like Croatian, but a foreigner who studies one of them perhaps would not say the same because there would be a lot of things he wouldn't understand, especially if he only began studying the language. I used to correspond with one Russian pal who would often get confused if I'd correct his mistakes in Croatian (he studied Serbian). There ARE differences in vocabulary, even in some grammar aspects. Trivial example:
Serbian - Hoću da vidim, i.e. hoću + appropriate person
Croatian - Hoću vidjeti, i.e. hoću + infinitiv
However, vocabulary differences are greater than these meticulous grammar and syntax things.
Secondly, about the alphabet. In Croatia cyrillic is NOT used on regular level. In Eastern Croatia perhaps in some areas with major Serbian people. But no way for something official. Number of Croats knowing cyrillis is a big one, however, though I don't come across a lot of my age people who know it.
In Bosnia I think they use both. I've been recently in Republika Srpska, one part of Bosnia, and saw many signs in both.
I don't know how in Serbia, but I think cyrillic dominates.
In short terms, learn one language and both alphabets and you can get by everywhere in that area, though I strongly advise you not to speak Serbian in certain parts of Croatia and vice-versa, or to claim that they are the same language in some parts, especially in Croatia, because if you come across specific people shit can happen, especially if you're a foreigner who, according to some of nationalistically-orientated, has no rights to say so. They will consider this a provocation.