After some thinking and searching (in the boards), it seems that all (animal/vermin) familiar understand Common, and can understand other languages based on their Intelligence and their master linguistics skill.
Understanding Common
Familiar’s type become magical beast for (they are animal/vermin initially): “(…) but is now a magical beast for the purpose of effects that depend on its type.”
“Magical beasts are similar to animals but can have Intelligence scores higher than 2 (in which case the magical beast knows at least one language, but can't necessarily speak). (…)”
Intelligence: “Any creature capable of understanding speech has a score of at least 3.”
“The number of bonus languages your character knows at the start of the game. These are in addition to any starting racial languages and Common. If you have a penalty, you can still read and speak your racial languages unless your Intelligence is lower than 3.”
As all familiar have at least a 6 Int, they are able of understanding speech. They don’t have a racial language. I believe they are characters so get Common as a language.
Speaking
Only raven, parrot and thrush can speak one language, as a Supernatural ability.
There is no mention of understanding it, so it is either a bonus understanding and speaking or the familiar need to be able to understand in another way. I think the latter is correct.
From the wording of the ability, and the fact it is supernatural, I also believe it is the only language they can speak.
Gaining understanding of languages
When the master gain level, the familiar has a better Intelligence. Starting at Intelligence 12, the familiar will get a bonus language.
The familiar also gain languages based on the master’s skill.
This post is relevant on the subject, and help me realize that rank in linguistics give bonus : http://paizo.com/threads/rzs2pexy?Languages-for-familiars#20
Basically, there is no difference between having ranks and putting them, because the +3 for class skill is mentioned to be when having (“If the skill you’re using is a class skill (and you have invested ranks into that skill), you gain a +3 bonus on the check.”) or having put ranks (“You gain a +3 bonus on all class skills that you put ranks into.”). So when the linguistics skill mention “Whenever you put a rank into this skill, you learn to speak and read a new language.”, I believe it applies to having the ranks.
Issues
There is the problem with the raven/thrush/parrot understanding of a language.
The linguistics part seems to give the familiar the ability to speak, and I don’t think it is the intent.
I also think it is strange that a familiar can learn language other that the one its master know.
FAQ
This is how I would write these questions if they were in the FAQ.
Do familiar understand Common? Do they gain understanding with increase in their Intelligence? Do they gain understanding with increase based on the master’s Linguistics skill? What languages can be chosen for the familiar?
Do raven/parrot/thrush gain the understanding of the language they speak? When gaining understanding of a new language do they gain the ability to speak it?
