The issue is rather complex. Most lands that the Germans took in 1871 had been taken by France using force (though seldom violence) since the Thirty Years War, by when each of the small parcels taken lamented loudly but in vain against being annexed by France, the military superpower of the era. Ironically, the oldest French possession, the Sundgau, had been sold to them by Austria well before the Thirty Years War.
In 1870/71, the annexation of the Alsace was no issue. The French governemnt had expected that and accepted it. Also, Bismarck had no great choice here. His new southern German allies had made it clear that they would not accept the invasion gate of Straßburg in French possession.
The real fuss was about Lorraine and especially the area of Metz and the lands south and southeast of the town, inhabited by French speakers (not Metz itself which was a German language isle in French speaking territory).
Here the German General Staff with its desire to take the fortress Metz in possession superseeded Bismarcks aversion against more foreign speakers added to Germany.
Subsequently, when talking about Alsace-Lorraine, the Germans had "German" Alsace in their mind while the French had "French" Lorraine before their eyes.
Had the population been asked (which never was the case) - in 1870/71 they would perhaps have voted to remain with France.
In November 1918, however, in the face of German defeat and approaching French troops, the A-L Landtag voted for independece of A-L. A fact of course completey ignored by France, but perhaps the only demonstration of public will telling what they really wanted that ever was possible in these contested lands.
While during German times, the French speaking population had received basic school training in French, after 1918, the French banned German and confronted children that had grown up in a German speaking environment with school instruction exclusively in French. After 1945, German was even forbidden in Kindergarten.