When French Canadians sing "Oh, Canada" do they sing it in French?
Oh, yes. In fact, it was originally written this way. The English words were written later.
Checking out the details on Wikipedia, I find that the song was originally written for a St. Jean-Baptiste Day (!) ceremony in Quebec in 1880. The English lyrics used today were written by a different author in 1908.
The French lyrics mean:
O, Canada!
Land of our ancestors,
Your brow is wreathed with glorious flowers!
Just like your arm carries the sword,
It also carries the Cross!
Your history is an epic
Of the most brilliant exploits.
And your valor, in faith steeped,
Will protect our homes and our rights.
Thus, the French version of Canada's national anthem is explicitly Christian, but political correctness is only required in Canada when speaking English.