I believe that because Sulivan himself cast a spell or something of the likes to make him communicate, because he also can t understand Sulivan off the bat.
Its only in the anime (went to double check)
Yeah, I think they retconned using the anime and then made it cannon by including it in this arc. (I just reread the manga up to before the music festival to prepare for the anime this season )
Hell YEAHH
ε=ε=(ノ≧∇≦)ノ
Wait holy shit I didn't even realize Iruma could speak demonize the first time while Merize couldn't communicate with Henri. I need to pay attention bruh