I guess it could work either way. I mean, Nagini could be obeying Voldemort by virtue of being a well-trained pet, the Basilisk for... whatever reasons the Basilisk does anything for, and Malfoy's summoned snake might listen to Harry because it's inclined to grant random non-difficult favors when asked. None of those seem any less probable than snakes winking, talking, having theory of mind, speaking in ridiculous hisses or knowing Spanish. In fact, none of the snakes in this series seem like snakes at all, so I'm not sure what my priors are regarding them.