hear me out: that just might be, really far stretch I know, but it just might be because the western weekend formed out of the Jewish Sabbath, which was adopted by Christianity. However it is not anymore the justification for having it. The only reason Saturday and Sunday are the weekend is because nobody bothered moving the date after the religious meaning was largely lost on the general population. Religion in Europe is in steep decline, unlike in certain other parts of the western world.
France has a population of ~40% Atheist/Agnostics. If you seriously think Christianity dictates the laws in France you are delusional.