Quoting: southern
They had also a lot of literature and lots of contacts with West,so maybe their language became more complicated.
I am sorry, but your theory is rather silly.
"Complication" or "simplicity" of a language (as perceived by you) would be a sign of civilizational advancement related to contacts with the West as opposed to the primitive, rough East. Am I right?
1) Poland, the Czech Republic, Slovakia are not eastern Europe. They are central Europe. Check your map, please.
2) The older the language, the simpler it tends to get. Look at English. Its early forms were a nightmare complete with 4 forms of irregular verbs, inflections and what have you. The relative complication of Slavonic languages stems from the fact that they are a bit younger than English, in other words, they had split from their joint Old Church Slavonic stem about 500 years later than Anglian or Anglo-Saxon had from whatever it was that Germanic tribes used to speak before they spread around Europe.
3) Russian is not a primitive language. Quite the contrary. I spent 4 years of primary school and 4 years of secondary school learning it and had a very strict teacher. The best I ever got was a B.
4) Please do realize that central and eastern Europe was not a barren desert of cultural and linguistic desolation even under the communists. We share our history and culture with you and it is extremely humiliating to be treated like a weird species of subhumans aspiring to the unattainable golden West. It's time to wake up and smell the coffee.