I think it's because kdramas don't usually do it. It's the same when they make a kdrama based on the japanese…
one of the background translators in the scene speaks perfect mandarin though, they definitely could have dubbed proper chinese over this guy , it's actually painful
That’s true but it’s also important to consider that the show has 40 episodes. I would guess they have to play it slow or else there wouldn’t be much of a second half of the show if they’d have gotten together already.