Does the Canadian prime minister have to speak french?

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.