If M6 felt that its customer base wouldn't grasp the significance of the '8' in the title then they could have gone with the film's alternative English title, "The H8ful Eight".
Hey @augustus .
You're thinking of it with a English point of view : if French people see the word "H8ful", their first thought will not be 'eight' for '8', they'll be thinking 'huit' (= eight in French), thus there's even more confusion.
One theory i have for the translated title is like often, it comes down to pronunciation. I'm saying the title in my head with a thick French accent and yes, you will find many people not really familiar with the English language who will pronounce "hate" the same way as "eight". It could be tricky to remember and/or say it when you are at the box office asking for a ticket.
I don't think it's got something to do with Inglorious Basterds. As you said, Inglorious Basterds was released as Inglorious Basterds in France. Of course people know Tarantino for that great film, but no one will make the connection between the English word "Basterds" and the french word "Salopards", other than they're both insults.
By the way, "bastard" is a lot friendlier in English than "bâtard" in French, in my view/experience at least... "you magnificent bastard" is okay in English, not its French translation ; "mon salaud" conveys the same friendliness - and "salaud" is kind of related to "salopard" (the feminine version of "salaud" is "salope").
The literal translation for "hateful" is "haineux". But Les 8 Haineux doesn't sound good.
But interestingly, the 1960 western The Magnificent Seven was released as Les Sept Mercenaires in France ("The Seven Mercenaries"), and also interestingly, the 1967 war film The Dirty Dozen was released in France as "Les Douze Salopards" (= The Twelve Basterds) . So Tarantino may have made a reference to these films with his title, and the French title translated it just like we did back in the 60s for those two influencial war/western films.
On an unrelated note, I've seen people refer to the UK/German Sicario steelbook as having the WWA art. It's not WWA, and the French steelbook isn't WWA either, there's no WWA possible for that film, it just happens Lionsgate exists in Germany and they could share the steelbook run, but there's no Lionsgate in France — Metropolitan picks up many of their releases, including Sicario and Hunger Games, also getting an upcoming Metropolitan steelbook with art that is different from the UK release (I mean there's no Mockingjay I & II combo planned for elsewhere, amazon.co.uk has a complete series steelbook though). Universal, Warner, Paramount have French branches, StudioCanal also shares steelbook print runs between UK and France for new films (Shaun the Sheep, Paddington, Macbeth), but not Lionsgate .
EDIT : There's a great article (in French) about the fact that for the first time with a Tarantino film, the title is translated. It confirms my reasoning about the referencial title not to Inglorious Basterds but to the big ancien westerns. They see the literal translation of "Hateful Eight" as "Huit Odieux" (not "Huit Haineux" like I said before, I guess both are equally not very compelling for a film title). Also the article mentions that Tarantino's films bounce around between distributors: all of his films until Death Proof were distributed by TF1, but Inglorious was with Universal France, Django was with Sony, and now H8ful is with M6 for the DVD/Blu-ray market (SND for the big screen, and the article says they paid a pretty penny for the rights).