There is no constitutional requirement that a Prime Minister has to speak French. In fact there are hardly any requirements to be Prime Minister, as long as you are a Canadian citizen and of voting age (18 or older). So if an 18 year old somehow became leader of a federal party and won a majority of seats in Parliament, he'd be the new Prime Minister.
That being said, if you don't speak French then chances of your party winning any seats in Quebec, which has the second highest amount of seats after Ontario, are almost zero. They wouldn't be able to participate in the French debate or communicate with French Canadians at all.
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Some Canadian's Speak French because during the time of immigration to "The New World" settlers came from France, and England, who took over Canada. Most Canadian's are indeed English, however in the central part of the country they have larger masses of French speaking populations.
Because Quebec was once a French colony.
Togo speaks French because it was a French colony from 1918 to 1960
Since 1902, all British Prime Ministers have been members of the House of Commons as opposed to the House of Lords. As Winston Churchill was Prime Minister post 1902, he was a member of the House of Commons, not the House of Lords. http://en.wikipedia.org/wiki/House_of_lords
He spoke French, but also spoke Italian and English fluently.