That may be the case, I'll check around to see what I can find. So far, I've checked the Monster Manual, PHB, FAQ, and all four Rules of the Game articles on Animals. Any suggestions on where to look would be excellent.
The closest I found was page 7 of the Monster Manual. When describing intelligence, it says a creature with Int 3 or higher knows at least one language. But, I have not seen anything about teaching a language to an animal companion (either for or against it).