During the 17th century, when absolute monarchies flourished, France adopted this model of government thoroughly. King Louis the XIV's court was arguably the most refined in the world, and one of the most numerous (since absolute monarchy meant the undermining of the feudal nobility, which were "imprisoned" in the King's court). To hold so many nobles, he built the most luxurious palace in Europe, Versailles. There there took place lavish banquets on a regular basis. France was responsible of making diplomacy an art: when foreign diplomats were shown Versailles and its lifestyle, they were in awe, so French became the language of diplomacy (this was an early version of soft power). France was at that time also one of the centers of the Enlightenment, so French was also spoken in academic circles. Since both the fields diplomacy and academics were dominated by the upper classes, it became a requirement and a sign of identity for the upper classes all around Europe and the Americas to know the French language. These customs persisted after the French revolution, since France remained the world capital of the fine arts (most artists in the Western world dreamed of moving to Paris).

EDIT:Louis XIV, not XVI EDIT 2: Grammar