With all due respect, there is a very large difference between any ordinary citizen and a monarch or senior member of a Royal Family. In addition, a monarch tends to stick around for a far longer period of time than a temporary president and their fleeting position, so what they do, don't, can and can't matters a lot more.
The ability of Their Royal Highnesses to survive in Paris is not in question. It is far more interesting to consider that when they become King and Queen of Canada, an official French-speaking nation, where every political and social leader is required to at least attempt to use both languages, and where it is quite common in speeches and so forth to switch between the languages, it will be seen as less inclusive to have a monarch and consort unable to use the language.
Is it a requirement? No. Is it outdated to learn French because it was once the language of courts and higher classes? Sure, but that's not why a working knowledge of French is more important for the Windsors than for us here on TRF, or many other Royal Houses. It is more important because they're not the British Royal Family, they're the Commonwealth Royal Family, where French is the 2nd largest language, official in 10 nations. It's important because the Royal Family of Canada has to master both languages if it is to have enough legitimacy to not be seriously threatened by republican movements.
Much can be said about learning languages. Some people struggle more than others, but very, very few will be completely unable to. If you're motivated, you will almost always succeed. French is not particularly difficult for an English-speaker to learn, especially one with access to tutors and all kinds of assistance.
Obviously, there is a difference between fluency and learning phrases to be respectful when being overseas. When Her Majesty went to Ireland and greeted the president and guests in flawless Irish Gaelic, it showed more respect and healed more wounds than much else could had done. However, when the Queen or other members of the Royal Family is in Canada, they're not overseas, they're at home, in one of their realms, and speaking in French is one of the ways they show just that: They're home, and they're Canadian.