Your all forgetting one big point there are many animals within the beastiary that are able to speak like the Warg. If a wolf like animal is able to speak not only common, but goblin as well what limiting other animals if they have the intelligence. Plus, with animals that are able to get over 12 intelligence i say they can learn to speak a learn language on there own. Many creatures are able to speak within the beastiary that don't have the so called makings to, but till do. Example Unicorn, Dragon, phoenix, Nightmare, Hellhounf Nessian, any half celestial or infernal, Chimera (speaking from three different head non-human, Blink dog, Silvanshee and Aranea. Some even have a intelligence of 4. Also many races have animal heads and speak totally fine. I say the ranger has to teach them the language, much like teaching another ncp or pc to speack a language; or they have a magical way of inducing speech.
Plus, i like to have Ntanga found in the NPC Guide, a sentient ape rogue that speak a Abyssal, Common, Elven, Osiriani, and Polyglot; no one but five different languages.
COLLAR OF THE TRUE COMPANION is a item that also allows animal companions to speak located within the ultimate equipment book.