Brel was from a traditionally Flemish family, but from the part of Belgium around Brussels that adopted French in relatively recent times.
I first heard this track from the 1950s relatively recently - maybe 10-12 years ago, via Stuart Maconie on the radio. The melody and the build-up of tempo hits you at first, the latter leading to some quite spectacular vocal gymnastics.
But it's also worth trying to follow the lyrics if you speak any French at all. It's full of puns and plays on words.