The sole official language in Lithuania as well as the language you will hear the most is Lithuanian (native to some 85% of the population and spoken by 96%). With millennia-old history and struggles for its survival, Lithuanian language is very much a part of national identity.
Minority languages in Lithuania
The largest minority languages are Russian and Polish, spoken natively by 8,2% and 5,8% of the population respectively.
Russian native speakers live primarily in cities. They include not only ethnic Russians but also many Belarusians, Ukrainians, Jews and other ethnicities common in the former Soviet Union, collectively referred to as Russophones. Some "Russophones" speak another language natively but still speak better Russian than Lithuanian or don't speak Lithuanian at all. Not speaking Lithuanian is common among the elder generation of Russophones. That's because during the Soviet occupation (1940-1990) knowledge of Lithuanian was not required and Russian was the lingua franca (unlike Latvia or Estonia, Lithuania gave its citizenship to every person who resided in Lithuania at the time of the collapse of the USSR, regardless of their knowledge of the official language).
Polish is spoken mostly by the ethnic Polish minority in the southeast Lithuania (including Vilnius). In some towns there Polish is actually the majority language.
While there are other minorities in Lithuania it is very rare to hear any other language than Lithuanian, Russian or Polish spoken in Lithuania by locals in their own conversations (rather than when talking to foreigners).
Minority languages are used extensively by the minorities in question and this is promoted by the government. For instance, there are many public schools where either Russian or Polish are used as the medium of instruction. However, any official government texts (e.g. laws or street names) are Lithuanian-only.
Foreign languages in Lithuania
Spoken by some 70% of the population, Russian is still the most popular second language in Lithuania, although this is declining. The Russian language was both mandatory and ubiquitous during the Soviet occupation (1940-1990), making virtually everybody in the older generations (i.e. those born ~1980 and before) fluent in it. Nowadays, however, many ethnic Lithuanians regard the Russian language as a "colonial leftover". Only some 40% of the kids learn it. Likewise, Russian public inscriptions have been gradually removed or replaced by English ones (although the Russian language may still be seen on an occasional old plaque or in unrenovated museums). On the other hand, private hotels and restaurants have Russian menus and employ Russian-speakers to cater for numerous Russian tourists. While to some ethnic Lithuanians any use of Russian is a reminder of the tragic history, some others enjoy Russian music and media, claiming that "culture and politics should not be mixed".
English is the most popular foreign language to learn today. It is spoken by 30% of total population and 80% of the youth. Older generations are unlikely to speak English however as very few schools taught it seriously under the Soviet occupation. Today English is the language Lithuanians expect foreigners to know, so it is widely used in modern museums, hotels, tourist signs and city/resort restaurant menus. As the "top language" of the "prestigious West", it also became fashionable for some key local trademarks and popular songs.
It is relatively rare for non-Poles to learn Polish as a foreign language. It is not taught as such in schools. However, non-Polish people from areas with strong Polish presence may have some knowledge of Polish acquired in day-to-day life, making 14% of Lithuania's citizens fluent in Polish. Polish signs for tourists are available in the areas most visited by the Polish tourists, namely the Vilnius region and the borderland. Polish-inhabited areas have Polish cultural events and media, although they are mostly aimed at the local Poles.
German (spoken by 8% of the population) held popularity as a 2nd foreign language (after Russian, instead of English) under the Soviet occupation as the Soviet Union had close ties to East Germany (but no close ties to any English-speaking country). Today German usually competes with Russian for the place of second foreign language in school (after English). In the formerly German-ruled Klaipėda region restaurant menus and other information in German is more common due to catering for numerous German tourists there. In terms of cultural impact, German is far behind English, Russian and Polish, however: there is no local German media, few cultural events, few books for sale and so on.
Fifth and sixth foreign languages by the number of speakers in Lithuania are French and Spanish, but they are spoken only by some 2% and 1% of the population respectively.