Dear friend, before the civil war in former Yugoslavia all the languages you mentioned were only the one :Serbo-Croatian. So, you can chose to learn only on of them and you will be able to understand and use the others too. What could be the most difficult issue in learning these languages.? The declination and conjugation of the verbs, in my opinion, but I am not a language professor and my attitude toward this matter could not be valued enough. Anyhow, if you need some help, just let me know. Kamenko