haha, this is actually one of the things that confuses me about cartoons - how even characters like aliens can speak good English
-snip-
@The dog from UP

There is a thing in his collar that translates his thoughts and barks into English.
Also the Toy Story one could be about how they have watched people talk around them and then learned to speak themselves.
Most others are about them being a creature that speak in their own language that has been translated to English for viewers sake. Wall-E is a robot with no vocal chords or any kind of speaking device, so i wouldn't be surprised if he couldn't talk.