czech is really very difficult to learn if your non.-native speaker
Serbian and Polish are similar.
Little off topic
: Well, you know... I have a friend who comes from Serbia, and I always understood when she speaks in Serbian... Or do you mean the grammar is very hard? 
I think Ally meant that if you're english(for example) it is hard to learn those languages. Pronunciacion & spelling is so different than english, german, etc. I think(though I might be wrong) that when you're speaking in Bulgarian it's easier to understand those languages. I am from Poland so I find Czech quite easy to understand, I can even speak a little(very badly though).
btw. I would love to learn Bulgarian, Serbian, Hungarian and so many more!