@WolfOfEve: Well I mean... the definition of a talking animal in real life is pretty shoddy. An animal is not required to understand the sounds they make or say something remotely intelligent for that matter. All they have to do is make sounds that sound like words. In practice, only the most extremely basic form of communication has been established. Things like single words.
The other issue here being that humans have very unique vocal cords. Any Pokemon without those is going to be physically unable to speak properly regardless of how well they train. (Once again. Meowth made reality his pet with sheer willpower. I don't even know.)
Sign language would be marginally more reasonable, but that raises an entirely different issue in that most people won't understand sign-language! (There's also plenty of pokemon that are physically incapable of sign-language as well.)