That's an ability. It's only real use is in dialog. If you attempt to cast it you see an animation but it doesn't do anything this way.
To learn you need a charisma of at least 16 to ask "My apologies, but I can't understand you. Would there be a better way for us to communicate?" or you need to speak with Hargrimm and ask him how he understands her. Then you can ask Stale Mary about his response.
--- No locked doors, no windows barred. No more things to make my brain seem SKARD.