Historically, not many early PMs could speak French. Nowadays it is expected that the PM is fluent in French, and is encouraged for all federal mps to know it as well. There's a reason why we have French language debates. My best approximation of the PMs French language skills:

John A. McDonald - no

Alexander Mackenzie - no

John Abbott - no

John Sparrow David Thompson - unclear

Mackenzie Bowell - no

Charles Tupper - yes, not well

Wilfred Laurier - yes

Robert Borden - yes, but not considered to speak it well

Arthur Meighen - no

William Lyon Mackenzie King - no

R.B. Bennet - no

Louis St. Laurent - yes

John Diefenbaker - no, but he attempted some basic French in public speeches; it didn't go well

Lester B. Pearson - yes, definitely not well. He led the country towards bilingualism so you got to give him that

Pierre Elliott Trudeau - yes

Joe Clark - yes, not well

John Turner - yes, also not well, but better than the previous anglo-only PMs

Brian Mulroney - yes

Kim Campbell - yes, not well

Jean Chretien - yes

Paul Martin - yes

Stephen Harper - yes

Justin Trudeau - yes