. I'm generally quite good at pronunciation, but with French, I need to be around it for a little while and listen to it to get my my pronunciation sorted out. If I had to read a speech in French right now, it would come out sounding a mess.
It would be nice if William could speak French but, IMO, doesn't affect his "suitability" for the position. If it were a real issue, I'm quite certain that someone would be making sure his French skills were still up to snuff.
I agree that French has a tough phonology. However, although most native English speakers don't realize that, so does English, as English actually has a large inventory of vowels, many of which are completely unpredictable from the spelling of the word. Nevertheless, many non-native speakers manage to speak nearly flawless English.
I guess the problem is not that native English-speaking people are not "naturally" good at foreign languages, but rather that they don't bother to learn a foreign language (because they think they won't need it anyway), or, when they do learn one formally, they never get to practice it after they leave school.