There are over 400 million Slavs in the world. The Slavic languages are very similar, so people from different Slavic countries can have a basic conversation without needing to learn other persons language. As a Polish speaking person I tried it with Czechs, Slovaks, Serbs, Bosnians, Ukrainians, Russians, Slovenians, Bulgarians. It works. Much more difficult with some than with the others, but there's a common origin of all of those languages. There's also been a few artificial languages created for Slavs to intercommunicate. One of them is slovio.com