In their original state the langues d'oil, superficially at least, had just as many differences between them as the languages of the stem duchies in Germany. Language is not going to be the make-or-break factor in switching the fortunes of royal authority in France and Germany. In Germany, royal authority collapsed because the monarchy lost the key power struggles with the Papacy, the Italian city-states and the regional magnates in Germany. With the benefit of hindsight it's pretty clear that the Hohenstaufen emperors overreached themselves and pushed far beyond the practical limits of their authority, especially in Italy, with bad results.
As for the French kings, they did not have the burden of having the greater part of their legitimacy dependent on the support of the Pope in Rome. Nor did they have the capacity to overextend themselves so dramatically given that the actual power of the Capetian kings in the early medieval period was incredibly limited. On the contrary, it was the French regional fiefdoms that became overextended and vulnerable under the Angevins...
I suppose if the roles of West and East Francia could be reversed right at the beginning, with West Francia coming under the influence of strong kings and securing the imperial crown, while the East drew the 'short straw' of weak kings whose influence barely extended beyond the borders of their personal domains, you might be able to reverse the roles right down to the early modern period.