Your English is fine Dusan, and I'm sure you're pleased to hear that from another non-native speaker, meaning I can't really judge at all.
My English always got me through any English test with ease, I've never picked up a book, so yours should be more than good enough to pass a test aswell.
We use English without much trouble on a forum like this and doubt our own ability, but when you go on to talk to mates of yours or even students who are preparing for the same test, you find out how shockingly bad alot of people are at it.
I'm thinking about learning another language aswell, but haven't decided which one yet.