It was a commercial slogan for a toilet cleaning agent. An English equivalent would be "mr clean recommends you use mr clean". Today, it is used to point out when someone tells you they themselves delivered good work. Anytime you'd use the meme of Obama giving himself a medal, you could use this phrase.
Actually the existence of the French name is confusing for English speakers as although many would know enough school French to get "canard" == "duck" if prompted, in English this word ends up meaning some sort of fabrication or hoax, apparently from an old French joke where somehow that punchline of that joke was popularized in England so long ago we don't have records. Human culture is weird.
So the first thing an English native might get from "Canard WC" is probably hoax toilet, I expect the duck imagery of the product would bring the "canard" == "duck" meaning across though.