I think the only animal familiar that can speak per PnP rules is the raven (a nod to Edger Allen Poe, I suspect). Of course, the regular feat for familiars only has small animals and such, not pixies, panthers, mephits, etc. I looked up the Improved Familiar feat. It doesn't give any indication that the listed familiars there would gain the ability to speak, but many of them already can speak.
So, I would say, unless it's a raven or a creature already capable of language, it cannot speak.
((Note: As of right now, this is my opinion and not necessarily a DM ruling.))