Regarding Serbian, Croatian, Bosnian, Montenegrin (done in order of population size)... native speaker here. These are four names for one and the same language - in a sense that American, Canadian, British, and Australian are the same language. What one says they speak is usually indicative of the nationality they choose to belong to. Those that say they speak Serbo-Croatian are usually those that in some sense identify as Yugoslavian or "undeclared" with respect to nationality. Curiously, Yugoslavian (meaning south slavic) as a language never officially existed, although this would probably best describe it.
In ex-Yugoslavia there is a rather pronounced distinction between citizenship and nationality. In Bosnia and Herzegovina (BH) this distinction is actually enshrined into the constitution. Moreover, in BH all official documents must be translated into 3 constituent languages. As a funny example of this see how cigarette packets look in BH [1]. In all three instances they say exactly the same thing. NOTE: Cyrillic alphabet, as used in Serbia, has an isomorphic relationship to the Croatian latin alphabet (this same latin alphabet is official in the Serbian "language" too, although it is not called Croatian). To get an entertaining idea about how complicated things are in BH see [2] or [3].
In ex-Yugoslavia there is a rather pronounced distinction between citizenship and nationality. In Bosnia and Herzegovina (BH) this distinction is actually enshrined into the constitution. Moreover, in BH all official documents must be translated into 3 constituent languages. As a funny example of this see how cigarette packets look in BH [1]. In all three instances they say exactly the same thing. NOTE: Cyrillic alphabet, as used in Serbia, has an isomorphic relationship to the Croatian latin alphabet (this same latin alphabet is official in the Serbian "language" too, although it is not called Croatian). To get an entertaining idea about how complicated things are in BH see [2] or [3].
[1] https://www.globaltobaccocontrol.org/tpackss/share-pack/pack... [2] https://www.youtube.com/watch?v=KO0rEwwyB0g [3] https://www.youtube.com/watch?v=sCEYDdMue-8