Dungeons and Animals
In addition to languages known innately by your race and class, you can learn additional languages based on your intelligence score. Learning a language takes a significant amount of time studying, which can be done in between adventures and is accelerated with the help of tutors or quality study aids. The maximum number of additional languages you can know is equal to your Intelligence score – 10.
You may start your adventure with a number of additional known languages equal to your Intelligence modifier. However, if you wish to know an exotic language, it will cost twice as much. (For example, if your modifier is +2, you may know either two additional standard languages or one additional exotic language.) You may not learn secret languages such as Druidic or Thieves' Cant in this way.
Common – Language spoken by majority of civilized society members and known by many of the less civilized as well. It is well suited to express both simple and complex ideas.
Verdant – Language spoken by races with woodland origins. It's composed mostly of chitters and squeeks, and is more apt for conveying feelings and impressions than specific nuanced thoughts. It has several regional dialects, including Tundra, Broadleaf, and Tropical. If two people are speaking different dialects of Verdant, it is nearly impossible to convey complicated ideas to each other. However, all the dialects use the same script, so written correspondence can be used to communicate with more clarity.
Aridi – Language spoken by races with ancestors from deserts and plains. Most of the sounds are based in hisses, grunts, and growls. Depending on the species of the speaker, the accent can be hard to parse, as a snakekin speaking it would sound quite different from a lionkin.
Aquan – Language spoken by aquatic and amphibious races. It can be spoken underwater by those with gills, though it can be very hard to hear. Mainly consisting of glubs and smacks, it can be very hard to speak if one is not equipped with the right biology.
Auran – Language spoken mostly by feathered folk, using primarily the sounds of clicks and squawks. It's utilized most often to communicate at high altitudes when wind is rushing by and specificity is impossible, meaning it carries quite well.
Abyssal - Language of demons and evil planar outsiders.
Celestial - Language of angels and good planar outsiders.
Deep Speech – Language of inscrutable monstrosities and horrors.
Draconic - Language of dragons, as well as lizardfolk and similar monsters.
Goblin – A language spoken by goblins, hobgoblins, orcs, giants, ogres, and other similar monsters. It's not clear to observers if this is actually a fully fleshed out language, but clearly these creatures are communicating with each other so there's probably some system in place.
Infernal - Language of devils and creatures that have a hellish affinity.
Primordial - Language spoken by elementals and creature that hail from the elemental planes of water, earth, fire, and air. There are four dialects of Primordial, one for each element.
Sylvan - Language of the fey.
Undercommon – A language used most often by subterranean peoples and many of the monsters that dwell beneath the surface of the earth.
Some exotic languages require specific magical biology to speak, and as such even if you fully comprehend the language, you may only be able to barely speak any of it.
Druidic - Language known only to Druids.
Thieves' Cant – Less a language and more of a code used by thieves and rogues to communicate simple messages by disguising them in seemingly normal conversation. Also has a system of written symbols that convey very basic information while appearing completely unremarkable to anyone not privy to the code.
There are myriad other languages that are specific to their respective species. Most intelligent creatures have their own system of communication that these use with each other. Some examples are Bullywug, Gith, Gnoll, Sphinx, and Worg.