The larger European countries are far less homogeneous than some suppose, and this may be more true of France than some others: as late as the First World War, fewer than half the native-born citizens of France spoke French as their first language (iTunesU; John Merriman). There were also disconnects between the language of the rulers and that of the people. In the case of England, until the 14th Century, the court and nobility of England spoke French; laws were written in French and Latin, not English. Scotland was split between English-speaking and Gaelic-speaking regions. Until the 19th Century, the court language of Austria was Italian The court language of Piedmont-Sardinia, the dynastic homeland of the Italian monarchy, the House of Savoy, was French.
Guillaume le Conquerant could probably speak comfortably with his liege lords, Henri I (King of France 1031 to 1060) and Philip I, and some of the aristocrats in France, at least those that weren't trying to kill him.