After wearing a Republican label for 40 years while thinking and acting more like a Democratic organization, North Dakota's Nonpartisan League last week officially took itself into the Democratic fold. Most active members of the N.P.L., closely aligned with the left-of-center National Farmers Union, will be more comfortable as Democrats. But the shift will cause real trouble for some of the league's leading lights who were elected to the office when the N.P.L. controlled North Dakota's G.O.P. organization. Most trou bled : North Dakota's cantankerous, caterwauling U.S. Senator William Langer, 69, a longtime (40 years) member of the N.P.L., who was elected as a Republican but often votes like a Democrat.