The extreme eastern part of interwar Czechoslovakia went to the USSR after WWII and is now part of Ukraine.
If the Germans were justified in taking back territories which had belonged to France since the late 1600s, what is to stop the Mexicans from demanding the Southwestern US back? (Of course many have been demanding it for many years.)
Slovakia was a Nazi puppet state, the Nazis used it to station their troops to invade Poland.
The fact that America would beat their asses?
The situations are different. The people in New Mexico, Arizona and California do not want to be part of Mexico. The Sudeten Germans did want to be part of Germany.
Re Alsace-Lorraine: the inhabitant there had begun to identify with France more or less after the French revolution. Before that French rule in Alsace had been relatively mild: most of these institutions which had been in the German era before the 17th century were maintained, among them the university at Strasburg.
Plenty of Alsatians and Lorraines began to identify with France after the revolution and Napoleon. Though this was more the case among the more educated parts of the population; the more rustic population still stuck to its culturally German ways and its German language.
So one could say that in 1871 the opinions of the locals were divided, just as they were not unanimous in 1918.