Whether or not in theory it should or should not be okay for a P.M. to be unilingual is moot. Because from purely PRACTICAL political considerations NO party would be wise to select a leader who is unilingual. Because no federal party serious about gaining power wants to risk alienating Quebec francophone voters by such an unnecessary way, unrelated to policy or platform.

But that's exactly what would happen if a party foolishly chose a unilingual anglophone as leader. The party would be politically dead-on-arrival in Quebec as francophones would in droves  reject a  party that would choose a leader that doesn't even seriously try to speak their language, one of the two official languages of Canada. French Quebeckers would  not be thinking re. unilingual anglo O'Leary, "He's our guy!" Just the opposite. They'd feel he (a native Montrealer, no less!) was rejecting them, so they'd reject him at the voting booth.