About any time it's shown Erma speaks, it's usually in some 'Eldritch tongue' that involves casting a spell, intentionally or not.
The only time we've seen Erma even being hinted as as speaking plain English was the page just before >>95499634
where she describes the plan to her friends.
Which ends up cutting out just as she gathers them up.
Being what Ertma is, it's also possible she's managing a form of telepathy we, the readers, cannot see but people around her can.