That's not so easy. First, besides "pure" literary languages there are lots of dialects.

For example, the essential part of Ukraine speaks "суржик" ("surzhik") -- some mixture between russian and ukrainian.

Also, as I know, in Belorussia there is "трасянка" (trasyanka), dialect consisting of belorussian, russian, polish and, probably, even some other languages. I don't know how widespread it is.