I think that the reign of French as the standard language of diplomacy extended well beyond the time of France as an effective global superpower. I believe that it was the openness of English to foreign influences, a quality actively opposed in the French language, that gave English the edge in the twentieth century. Having a world wide empire may be necessary to establish a world language but it is not sufficient. English has been about change since the days of the Danelaw. The language is a creole taking bits and pieces of every language it has touched and for this reason is more available to other peoples.