Knowing how to speak proper common should not always be tied directly to the INT score. It might be the case with humans and common, but with other races it should also depend on their background. For example, if a half-orc has an INT of 12, he can't automatically speak very good common, it should depend on how much time he has spent with humans, was he born and raised within orcs etc.
So a high INT will make him speak better or learn faster, but in most cases he will still speak in a retarded way, unless he was raised by humans from day one.