from the FAQ: (i suck at linkiness)
Intelligence: If my Intelligence modifier increases, can I select another bonus language?
Yes. For example, if your Int is 13 and you reach level 4 and apply your ability score increase to Int, this increases your Int bonus from +1 to +2, which grants you another bonus language.
Technically, Int-enhancing items such as a headband of vast intelligence should grant a specific language (in the same way they do for skill ranks).
If I have 17 intellect and add +1 at level 4, do I get another language and skill ranks as if I started the game with 18 intellect?
It grants a new language AND you get retroactive skill points, meaning you get the extra skill point for levels 1-3 also.
yes, and the skill ranks are retroactive.
What about skill ranks?
you retroactively add both skill points and get a new language.
sorry, I knew it increased skill ranks, but I didn't think you got the bonus language, so went and checked (good thing i did, i was wrong!). When i found an answer to that, just posted it and forgot about the skill side.