Poland was a country located on the European continent on Earth.
Famed 15th century astronomer Nicolaus Copernicus was born in Toruń in Poland. (VOY: "Future's End")
In the 20th century Poland was a member of the Allies in World War II, fighting against Germany. In a holodeck simulation of this war on the USS Voyager, the German character played by the Hirogen Karr therefore had served under Schmidt in Poland and under Rommel in North Africa before being assigned to Sainte Claire in France. (VOY: "The Killing Game")
By the 24th century, the New Berlin colony hosted the Mazurka Festival, which was a celebration of folk dance and music originating in Poland. (DS9: "The Maquis, Part I")
According to Star Trek: Starship Creator, Katherine Pulaski was born in Krakow, Poland.