I haven’t read the Frieren manga, but the show referred to its world’s demons as not actually speaking, but having learned to make human-like noises to elicit desired responses.

I have read ahead quite a bit, but it’s actually not relevant, because the show itself (which is pretty much identical to the manga) already debunks that.

Aura and her executioners are clearly very intellegent and even have an inner monologue. (One of the lines I actually dislike is when one demon asks the other “what is a mother?” And he just replies with “who knows?”. From what we’ve seen from the demons so far, they should be more than capable of having figured out how human reproduction works).