Nudist or “naturist” philosophy grew out of the health and fitness movement in Germany, in the early 20th century. Although “clothing optional” beaches and private lands were popular in Scandinavia and the German north coast, the warmer climes of the Mediterranean obviously had an appeal to enthusiasts.
Even under the socialist government of Josip Broz Tito, prime minister or president of Yugoslavia between 1943 and 1980, the coast of Yugoslavia along the Adriatic Sea had many unofficial nude beaches. (In 1970 when I was 11, only a couple years after Unbearable Lightness is supposed to take place, my family toured the Yugoslav coast and visited at least one nude beach. The most plentiful tourists there appeared to be Germans.)
Today, the individual states of Croatia and Montenegro are still known to tolerate “clothing optional” beaches.