There were polish soldiers that served in the German Army in WWI; Alexander Watson actually has a very good article analyzing them, called Fighting For Another Fatherland: The Polish Minority in the German Army, 1914-1918.

About 850 000 served in the German Army (mainly the Prussian Army) in WWI, making up 6% of the German Army's manpower during the war. Perhaps 100 to 130 000 were killed in action or missing presumed dead. They seem to have fought relatively well, at the very least loyally, throughout the war, though my readings about the German Army on the Somme has indicated that Silesian troops which fought there were viewed as suspect, and some middle-ranking commanders blamed them for setbacks on sectors of the front. By 1917-18, especially with support among the French for a Polish State, the Polish soldiers seem to have featured prominently among deserters, and subsequently as volunteers for the Blue Army, a Polish force lead by General Jozef Haller von Hallenburg.