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?