The Player’s Handbook on Race and Languages says
All characters know how to speak Common.… A smart character (one who had an Intelligence bonus at 1st level) speaks other languages as well, one extra language per point of Intelligence bonus as a starting character. Select your character’s bonus languages (if any) from the list found in his or her race’s description later in this chapter. (12)
The Monster Manual on Intelligence says
A creature can speak all the languages mentioned in its description, plus one additional language per point of Intelligence bonus. Any creature with an Intelligence score of 3 or higher understands at least one language (Common, unless noted otherwise). (7)
However, some creatures—usually those to which the game has assigned a Level Adjustment therefore designating them suitable as PCs like the grimlock (140–1), minotaur (188–8), and even the rakshasa (211–12)—possess a list of bonus languages. Others do not.
If the DM is designing an NPC creature by the book, and the creature’s description lacks a list of bonus languages, yet the creature possesses an Intelligence score of 12 or higher, does the creature still pick extra languages? If a creature does, can that creature pick any languages, including secret languages like druidic and githyanki?
For example, Book of Vile Darkness says, “Kythons speak a strange smattering of both Infernal and Abyssal, but only to each other, never to others” (178), and the kython description includes no list of bonus languages, but a slaughterking kython (181) possesses, by default, Intelligence 20. (Seriously! Final-stage xenomorphs are geniuses!) Does a typical slaughterking kython speak only Kython, or does it speak Kython and five extra languages?
Note: I know that as the DM I could just rule either way on this, but there are some far-reaching—if relatively minor—campaign ramifications that result from either decision, and I like to be consistent with printed material when I can be (even if the authors sometimes aren’t themselves). And although it’s been in the back of my brain forever, yes, it was the slaughterking kython for tonight’s session that finally inspired this question. So you know, given the chance, the slughterking’ll also speak Drow Sign Language, Deadly Dancer, Grell, Neh-thalggu, and semaphore battle signals. Obviously.