My knowledge of French is not sufficient enough to judge William’s French speech.
During my time at (high)school in the Netherlands I had classes (incl. tests etc.) in Dutch (of course
), English, French and German. As I had no feeling for French I dropped this subject as soon as possible
At that time there were some (high)schools were you could also learn Frisian (which is a co-official language in the Dutch province of Friesland), Spanish and Russian. I have noticed that it is useful to have an understanding of various languages.
When I am correct the knowledge of foreign languages was not a requirement for Dutch Universities. With the exception of the languages studies and courses which were in English or another language. For many studies it was an advantage when you had knowledge of English or German. Among others because of the languages predominantly used in the associated professional literature (and professional world).
During time I heard Queen Beatrix speak Dutch, German, English and French. As I am no native speaker, except for Dutch, I am not able to judge if she is totally fluent in all these languages. I assume prince Willem-Alexander also speaks Dutch, German, English and French. Several years ago both princess Máxima and prince Willem-Alexander said he was trying to learn Spanish (the mother tongue of Máxima).