The Troops on Vacation (1970) on Netflix

Now maréchal des logis-chef (chief petty officer) Ludovic Cruchot has married the rich colonel's widow Josépha and been retired early from the gendarmerie (French paramilitary police) with his whole St. Tropez (a fancy French Rivièra resort) unit, he finds life as a castle lord unsatisfactorily boring. When his former chief adjudant (warrant officer) Jérôme Gerber and report their mate maréchal des logis (petty officer) Lucien Fougasse is in therapy at La Pinsonnière from amnesia, the veterans all take him on a trip down memory lane, in -now illegal- uniform, only to discover he fakes it to enjoy easy life in the mental asylum. Nevertheless they can't resist sabotaging their much younger successors' promising attempt -by parachute- to catch their favorite pray, the elusive illegal nudists, but find Cruchot's car was stolen, so they masquerade as hippies to penetrate the colony 'undercover' and retrieve it. Everything seems lost with the new commander opening the hunt and both veteran wives deciding to have their own fun, when the 'flying nuns' turn up, whose convent now runs a holiday camp for boys, the five worst scamps being missing for a day, as soon turns out with a most dangerous prank plan...