The stamps are inscribed in Polish because the country was occupied by Poland.
From Wikipedia:
Quote:
Central Lithuanis was a short-lived political entity which did not gain international recognition. It was created in 1920 following the staged rebellion of soldiers of the 1st Lithuanian-Belarusian Infantry Division of the Polish Army, supported by the Polish air force, cavalry and artillery. Centered around the historical capital of the Grand Duchy of Lithuania, Vilna (Lithuanian: Vilnius, Polish: Wilno), for eighteen months the entity served as a buffer state between Poland, upon which it depended, and Lithuania, which claimed the area. After a variety of delays, a disputed election took place on January 8, 1922, and the territory was annexed to Poland.
It eventually went back to Lithuania where it is today.
All their stamps are simple but really cool, as the one showed by Simon.