Why? Just why?
"France goes Nazi if Germany wins the war" is such an overdone trope. And, if you look at French political history, rather less likely than Al Capone becoming the American Stalin.
France was very much aware that it was in a state of relative decline versus Germany. Both before and after WW1, there were those who advocated accommodating to German ambitions and seeking a German alliance rather than the British alliance that France decided on before both world wars. WW1 ending with Germany retaining even more of France is just likely to strengthen the pro-German lobby since siding with Britain and Russia would be clearly shown to have failed.
Then there is the political dimension - the far right just never got much traction in France, for a number of deep seated reasons. But the political machinery of the third republic really doesn't lend itself to radicals gaining power. Unlike Germany, where the constitutional seeds for Hitler's power were planted within the Weimar constitution, the French constitution was filled with impediments to any sort of rule from the center (that included rule by non-radicals - there is a reason why France between the wars struggled to achieve any sort of reform).
Further, Germany barely had enough power to start WW2 - if Hitler had been even a little less lucky, WW2 would have never started and instead we'd have just had a little regional war where Germany got the tar beaten out of her. France, even if the interwar period went very, very well, cannot expect to equal even OTL's 1938 Germany's relative power until the late 40s at least (and that is with an economic and demographic miracle that frankly strains credulity). Thus, even if France does go nationalist and try to start a war, most likely it ends up getting thoroughly thrashed by the surrounding powers without the war drawing in enough participants to be a "world" war.
It is more likely that Britain would start WW2!
fasquardon