PCs know languages mostly dictated by their Race, and possibly their Class and Background. They may also gain bonus languages from high intelligence. Here you will find a list of the most obvious choices for extra languages, and a short description of each language's place in the world. There are many more languages than these; even Giant Eagles have their own tongue.

Abyssal: The tongue of CE demons named after their plane of existence, The Abyss. Spoken by them, and learned by summoners, cultists and magicians for dealings with the demons. The literature is highly esoteric, dealing much with magic subjects and religion, but also hard to come by. Many places, owning a text in Abyssal is a serious crime.
Abyssal is the language of demons, of lies and destruction.

Celestial: Spoken natively by creatures of the upper planes, such as angels, Celestial is held holy by many religions. As such, it holds a particular place for ceremonies and uses in holy texts. This also reflects in the literature, dealing with religion and philosophy mostly.
Celestial is the language of Good, of hope and of healing.

Common: Originally risen as a trade pidgin language, but formalized and expanded by the old Empire before it fell. Spoken widely by good and neutral cultures, and not a few of the more savage ones as well. Most of the literature in this language is fairly new.
Common is a lingua franca and the language associated with commerce.

Draconic: Originally invented by dragons, and taken over by many or most scaled humanoid cultures, such as kobolds and lizardfolk. As such, the original tongue, changing slowly with the long generations of dragons, is considered a ‘high’ tongue, while the dialects derived from it are considered ‘base’. The literature is tiny, but sometimes deals with very sophisticated subjects like magic and metallurgy.
Draconic is the scaly language, often associated with pomposity.

Druid: The secret tongue of the druids is almost never written down, for fear it will be deciphered by outsiders. The study of the language is inextricably bound with druidic magic, so that one cannot practice one without the other.
Druid is a supremely specialized language.

Dwarven/Dwarvish: An ancient tongue of runes and difficult pronunciation, dwarves everywhere speak Khuzduhl. There are almost no discernible dialects, nor historical drift, as the teaching and speaking of dwarvish is highly ritualised and formal. Few non-dwarves get the chance to learn it, though it is not a secret tongue.
The literature is small, often found as runes on rocks, or as practical ledgers and manuals.
Dwarvish is a highly specialized language.

Elven/Elvish: Elvish is understood to include both Sindarin (the everyday elven tongue which even drow use a dialect of) and Quenya (the high elf tongue of old). It is spoken in many places, where elves of various kinds dwell, and it has the richest of all literatures. It is widely learned by magicians as the language that best deals with magic and spells.
Elvish is the language of history, magic and ‘high’ scholarly pursuits.

Giant: The language of Titans of old, chiselled down in the uncouth mouths of hill giants over the millennia, kept crisp and expressive by cloud giants, or reshaped and cast into fresh shapes by storm giants. The dialects of the various kinds of giant are quite disparate, although usually conversation can be carried through with patience and care. The literature is small, but almost as old and distinguished as elvish. It is very difficult to access for the average humanoid, however.
Giant is a language of mystery, dread and splendour.

Goblin: Encompassing both the formal and practical hobgoblin tongue and the hodgepodge of goblinoid dialects, goblin is similar in use to Common, except that it is the lingua franca of goblinoids and other cultures considered evil. The literature is almost entirely of hobgoblin make, and tends to deal with matters of the military and warfare.
Goblin is a lingua franca and the language associated with mercenaries.

Halfling: The tongue of the halflings, Kuduk, is uncomplicated and has many disparate dialects. Halflings make some effort to keep their language inside their own culture, and are not likely to willingly teach outsiders. The literature is tiny, focused on genealogy, farming and gardening techniques above all else.
Halfling is a highly specialized language.

Infernal: The speech of LE devils, and learned by summoners, cultists and magicians for dealings with the devils. The literature is highly esoteric, dealing much with magic subjects and religion, but also hard to come by. Some places, owning a text in Abyssal is a serious crime, and in many others it is considered seriously disturbing.
Infernal is the language of demons, of tyranny and power.

Orcish: Derived from goblinoid dialects (though orcs would likely deny it), orcish is a particularly harsh and unlovely language, with some influence from draconic. Orcs have spread just about everywhere, at least in small numbers, and this causes the language to have many barely compatible dialects. The literature is tiny, dealing mostly with religious texts on Gruumsh and greatly embellished accounts of the doings of great orcish heroes.
Orcish is a language of strife and hate.

Primordial: The ancient source of the four elemental languages, Aquan, Auran, Ignan and Terran, it is rarely a day-to-day tongue for anyone. Instead, it serves as a common language for the inner, elemental planes. Its literature is virtually non-existent, limited mostly to certain holy or ritual texts.
Primordial is a language of extreme ancientry, of the nearly forgotten and the barely glimpsed.

Sylvan: Heavily influenced by Elvish Sindarin, because the Sylvan creatures were first taught speech by elves in an age long past. They now speak a language all their own, lyrical or swift as needed. The literature is very small, and consists mostly of poetry and story-telling.
Sylvan is a language of wild nature and fey creatures.

Undercommon: Deep underground are vast, sprawling cave systems, where whole civilizations and wildernesses exist. Much as Common above ground, Undercommon is the pidgin trade language for underground cultures. The literature is small, as these races all have their own languages, and prefer to use them when possible.
Undercommon is a lingua franca of the deep world.