In the first film, it's established that Elizabeth can talk to animals due to moon magic. So, why are all the other humans able to talk to the mice? Because the Mice are more anthropomorphic than the other animals, to the point where they could be considered people, and therefore, can possibly talk human language.
Also, if the mice are like people, this means Ronny's comment about not being a "racist" to explain his attracted to a human, makes a lot more sense.
As pointed out by The Nostalgia Critic, while the screwdriver ban is confusing, it turns terrifying when you realize that one of the minor characters is a sapient screwdriver implying that Atlantis has been engaging in some good old fashioned Fantastic Racism.
Also pointed out by Diva, if Elizabeth is immortal and can never leave Atlantis, then how will her father the Duke feel about all this? He'll never know what happened to her, and it's likely he'll die believing she and Don Juan died in the submersible.