Russian language
In the 20th century, Russian was mandatorily taught in the schools of the members of the old Warsaw Pact and in other countries that used to be satellites of the USSR. In particular, these countries include Poland, Bulgaria, the Czech Republic, Slovakia, Hungary, Albania, former East Germany and Cuba. However, younger generations are usually not fluent in it, because Russian is no longer mandatory in the school system. According to the Eurobarometer 2005 survey, though, fluency in Russian remains fairly high (20–40%) in some countries, in particular those where the people speak a Slavic language and thereby have an edge in learning Russian (namely, Poland, Czech Republic, Slovakia, and Bulgaria).

Q: What pact contained many Russian-speaking countries?
Warsaw Pact