Curate, connect, and discover
Ok but what if ghouls didn't just automatically know English when they are first summoned and then usually their summoner and fellow ghouls teach them language.
Now imagine that because Phantom was summoned at such a complicated time for the pack (with Aether and Sunshine's eminent departure looming over them) that he never really gets taught how to speak.
Aurora pretty quickly bonds with the ghoulettes but because the ghouls (either accidentally or purposely) outcast Phantom he just only learns little bits that he catches in passing during band practice.
And they only notice when tour time comes around and they realise they've completely neglected this new ghoul and he can't understand a single thing anyone says to him.