The shortest distance between England and the continent of Europe is (across the Straits of Dover) a little over 33 kilometres or 18 miles.
The distance between the Netherlands and Spain is certainly more than that yet if and when King Felipe and King Williem-Alexander meet, (and they have done on several occasions), they can converse in Spanish and English together. Would William be able to converse informally in German, Dutch or Spanish with King Willem? No, of course not!
I would just say that the world is getting smaller each year and although English is indeed an international language, it's not very admirable to remain mono-lingual or to show off your terrible French when you are the future leader of a nation, and perhaps Commonwealth, in which many, many languages are spoken.
Even if foreigners to whom you speak are kind enough to speak to you in your own language, IMO a whole other world, in literature and the arts for example, opens up to you when you have another language under your belt. Why rely on other people's good will?
William would greatly benefit by knowing French, IMO, not least because he wouldn't then humiliate himself in front of French Canadians when he attempts a speech.