Poland was a country located on the European continent on Earth.
Famed astronomer Nicolaus Copernicus of the 16th century was of mixed Polish-German origin. (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 Lunar city of New Berlin annually hosted the Mazurka Festival, which is a celebration of folk dance and music originating in Poland. (DS9: "The Maquis, Part I")