As per RAW the familiar understands any language the mage is able to speak. Now given that the core rules mostly applied to magical animals, the expanded ruleset (which includes familiars capable of speech on their own) beckons the questions: Does this also work in reverse? Does the mage understand any language the familiar is able to speak? And if so, does this work with faerie familiars who have the Faerie Speech virtue?
RAW doesn't and I don't see how this would be good storywise. Maybe if the relation was perfectly symmetrical and both were familiar of the other XD.
I agree with Tugdual. Note that the rules list very specific benefits for the mage and very specific benefits for the familiar, and they nearly all differ from each other. As the familiar is the one granted some sort of intelligence and not vice versa, it stands to mind language would go in the same direction as it does. I see no logical reason to even consider it working in the other direction based on what's written.
I could see a generous storyguide allowing it under Shared Powers (from the Faerie Magic version of the familiar bond). But I still wouldn't rule in favor of it there.
Chris
It's a good idea for original research, though.
What about when your extant familiar goes off and learns a language you haven't picked up yet? Is there an enchantment you could put into the familiar bond that lets you use each other's language knowledge? Other skills?