The last Prime Minister who could barely speak or read or write or understand French was John Diefenbaker. Though in truth, Im not too sure whether Lester Pearson was fluently bilingual. Perhaps another reader might advise. But since 1968...this nearly a half century, our PrimeMinisters have been bilingual. Jean Chretien may not have had perfect english or even perfect French, but he sure as hell knew what was going on in both languages. 

There is of course nothing to prevent  a person from pursuing the leadership of a political party or of the country being fluent in only one official language.But in this day and age, i think it would be very difficult for that person to win. For very valid reasons. It doesnt mean that this person cant contribute, be for example, a senior advisor or member of parliament. But just as with the Governor General, the Prime Minister should be able to speak to all Canadians, and both listen and hear what people are saying in two official languages.