Reading up on languages it looks like characters are supposed to start play with their base languages plus one language per point of intelligence modifier.

Looking at monster stat blocks however this does not seem to be the case.

A Planetar has an Int score of 22 (+6) yet it only speaks 4 languages (Celestial, Draconic, Infernal, and Truespeak).

Dragons are no different. An Ancient Blue Dragon has an Int score of 20 (+5) and knows 6 languages. Dragons speak Common and Draconic starting out so the dragon should have at least one additional language.

Am I missing something here?

They don’t, no. I mean, arguably, they could just have a really short list of options for bonus languages—those lists aren’t printed anywhere for most (any?) monsters, and it could result in not being able to learn new languages even if your Intelligence would otherwise entitle you to some—but really, the answer is just no. They do not follow the same rules, they just know the languages they’re written up with.

They do not.

Answering this same question at Paizo's messageboards, James Jacobs (Paizo's Creative Director) said that bonus languages are arbitrary:

Monsters should always have at least one language listed if they have an Intelligence of 3 or higher; the number of base languages a monster knows, though, is relatively arbitrary.

Monsters with an Intelligence of 2 or 1 or those without intelligence at all NEVER know languages. This includes all vermin and animals. Communicating with them requires magic (like speak with animals) or other things like the aforementioned Handle Animal skill or wild empathy.

PC ≠ Monsters

Monsters are not created by the same rules as you build an PC, so no, you cannot apply any PC creation rules to monsters and expect that they will be followed.

A summary of the moster creation rules can be found here. The intro already indicates normal straightforward rules apply (emphasis mine):

Creating a monster is part science and part art. While most monsters follow a general patern of their overall power and abilities as related to their Challenge Rating(CR), there are many exceptions.

Languages are not mention in the source but would follow under Step 10:Details which says (emphasis mine):

Now that you have all of the creature's statisctics sorted out, t's time to fill in all the details - such as name alignment, space, reach , environment, and ecology - as you see fit.

In other words figure out what you think is appropriate for your monster and go with it.