Michal:Origunally, Russian was THE language but spread with the tribes many centuries ago.
"All Slavic tongues are believed to have evolved from a single parent language, usually called Proto-Slavic, which, in turn, is thought to have split off much earlier (possibly c.2000 B.C.) from Proto-Indo-European, the original ancestor of the members of the Indo-European language family. Proto-Slavic was probably still common to all Slavs in the 1st cent. B.C., and possibly as late as the 8th or 9th cent. A.D., but by the 10th cent. A.D. the individual Slavic languages had begun to emerge." (Britannica Concise Encyclopedia)
I learned Polish at the Army Language School years ago. There were other students taking Czech, Serbo-Croatian, Slovenian, Russian, etc. I found that I could better under those other Slavic language, then they could understand Polish.
For example, friend in Czech is přítel, in Slovak it is priate, in Slovene it is prijatelj and in Polish it is przyjaciel, but pronounced as if it is pszyjaciel.
The other could not under przyjaciel as being přítel or priate or prijatelj. The Psz sound was lost to them because they expected pr.