Αδελφέ Γιάγκε! Αδελφέ Γιάγκε! Κοιμάσαι; Κοιμάσαι;
why not : "who sleeps?"
That's not the meaning in English. "Who sleeps?" would be used in a context such as: "Who sleeps in the big bedroom." it is a general meaning of sleep. Without context "Who is sleeping." is the only correct translation.