Share Language (Utopian).
It's only in the process of actually untying the spell that Esta realizes/remembers -- he has not cast this spell literally since his days as a 2nd-circle -- that Esta could now in principle use it to grant more than one language; indeed, he could use it to grant three.
In that split second, Esta decides --
-- to grant Merrin only Utopian, because that was his original plan and Esta does not have any time to think about it.