By far the most widely used Eastern European language is Russian. In addition, it is the most popular Eastern European language to learn, not least of all because it is taught in universities and some high schools. Other languages, like Polish, Czech, or Romanian may be taught only in larger universities and offered rarely – and even then, only basic-level classes may be available. This is not, however, a comment on the popularity or usefulness of Russian throughout Eastern Europe, and should not be taken as such. Using Russia outside of Russia when traveling through Eastern Europe should only be done with extreme caution, even if your only two languages consist of English and Russian.
Russian may be used in Russia, some parts of Ukraine, and anytime you are invited to speak Russian with a non-Russian citizen. Since the fall of the Soviet Union, citizens of Poland, the Baltics, Romania, and elsewhere choose to speak their national languages. This is especially true for generations that remember the Russification process – speaking Russian is a reminder of that time when their own language may have been outlawed and they may have been forced to communicate in Russian, whether or not they had any grasp of the language when the policies were enacted.
For example, if you know only a little Polish and are fluent in English and in Russian, a Polish individual may prefer you to speak English or broken Polish over Russian. Younger non-Russians may not place such heavy stigma on speaking Russian, and if this language is preferable over English, you will certainly be invited to speak in Russian. It is best not to assume that this language is an acceptable one to use.
Russian language can still be useful in Eastern Europe even if you are not traveling in Russia. Russian, as a Slavic language, shares many similarities with other languages in the region. Therefore, reading signs and menus may be less daunting if there is no English translation. There are also many Russians living outside of Russia who will have no reservations about speaking their language with you. After all – Russian language, even though it was required by Soviet officials, is not outlawed in non-Russian nations. But you may insult a few people if you go around speaking it with abandon.