I lived in Belarus for some time, and my wife is from there. We spoke only Russian while in the country, but most signs, public transportation announcements ("next stop", etc.) and documents were in the Belarusian language.
I've learned very basic amounts of Belarusian, and I'm wondering what the linguistic similarity (language family, etc) is between it and the Russian language. It seems to me to be more mutually understandable with Polish, but it has quite a few "Russian-sounding" words as well as a seemingly similar grammatical structure.