Largest number of mother tongue speakers in Europe
Russian has 144 million mother tongue speakers and a further 110 million people speak Russian as a second language, making it one of the largest languages in the world and the largest in Europe. Russian is a member of the Indo-European language family. Within that family it belongs to the Slavic group of languages.
In Europe there are significant numbers of Russian speakers in e.g. Estonia (+/- 25% of the population), Georgia, Latvia (+/- 28%), Lithuania, Moldova and Ukraine (+/- 30%).
Source: Wikipedia