Russian belongs to the Slavic group of languages, which is divided into West Slavic (Czech, Slovak, Polish, and Sorbian), South Slavic (Bulgarian, Croatian, Macedonian, Serbian, and Slovenian), and East Slavic (Russian, Ukrainian, and Belorussian). Learning Russian opens the door of opportunity for facilitating the learning of other Slavic languages, those spoken by a little over 300 million people from Eastern Europe to Northern Asia. Take a moment to look at the map of Slavic languages below (special thanks to Wikipedia).

Link: Why Russian?