So.. was it the mistake of the translation team, or the mistake of the manga writer, that had the female leads know Diablo's name before he stated it? (Page 17)
I just checked the first episode of the anime, and it utilizes a much more natural "him" pronoun, rather than a proper name.
Likewise, the core lore of the story has him as an unknown summon, so it isn't as though there's any basis for them knowing the name.