That is correct; this is an international film with many English-speaking cast members (Fonda, Albright, Booke, Howard, etc.), and thus, both versions are dubbed. Clement made many international films with alternate soundtracks prepared: Rider on the Rain, The Deadly Trap, And Hope to Die, The Babysitter, for example. Rider on the Rain has Bronson speaking in the English version, while blacklisted director John Berry (resident in France) was the voice dubbing Bronson in the French version where the rest of the cast speaks French, e.g., Eva Green's mom Marlene Jobert who is the star of the film. The concept of one version being official and the other dubbed makes no sense for these types of films, although the quality of the results is what counts (e.g., Burt Lancaster in The Leopard -folks prefer the Italian track, not the English-track which has Burt's familiar voice).
It appears to me that the film was originally shot in English, but I don't know if it's true. So did Alain Delon speak French or English in the original?
"WHY DIDN'T YOU STARVE FIRST?!" - Humphrey Bogart, 'Dead End' (1937)