Tongues of Man

• Citizens of the First Empire spoke Old Imperial (Latin), and it is still the language of choice for many of those who value education or pretention (depending on whom one asks).
• Subjects of the Kingdom of Luxoria speak Luxorian (French). Since Urbois is one of the great new centers of magical learning, many mages in far-off lands will speak Luxorian.
• Free men, women and lizard-people living in and around the Blood Sea speak Libertine (Spanish). The Blood Sea is home to raiders and traders whose ships range far and wide. Merchants in far-off lands are likely to know Libertine.
• The Southern Highlands are home to a society known for slaves and mines. Blacksmiths and slavers are likely to know a smattering of Valarian (Italian).

Other Mortal Dialects

• Elves speak Elven. They've got long enough lives that their language hasn't fragmented within human memory -- perhaps not even within human evolution.
• Dwarves speak Dwarven. Like Elves, they live a long time. Unlike Elves, they tend to write things in stone. So, their "texts" are preserved, and their language is fairly static.
• Free Orcs speak Orcish. They have many words for honor-type things. (I might steal from Japanese if the PCs ever visit there.)
• Goblinoids speak Goblin, a corrupted patois of Sylvan and Abyssal. It's full of idioms, most of which translate literally to different types of pain or torture.
• Giant features lenghty, verbose constructs allowing exemplary specificity, however preternaturally grandiloquent utterances promote soporific verbiage, consequently producing circumlocutious phraseology. (Unsurprisingly, it is spoken by Giants.)
• Gnolls have no language of their own. They are the ritually tainted (and transformed) followers of a Demon-Prince, so it's a good bet that some speak Abyssal.
• Lizardmen and Kobolds speak Draconic (see below), and possibly local mammal languages.

Languages of Power

• Mortals cannot lie in Celestial, and Celestials do not lie, but Devils are not so restricted. Demons cannot understand Celestial, but they know it causes them pain.
• Oaths spoken in Infernal are eternally binding, sometimes in horribly literal ways. (Some cultures have dabbled in using Infernal for wedding vows, with dark and disasterous results.)
• Mere knowlege of Abyssal by mortals saps their sanity. -2 to all Will saves vs. Tanar'ri, and Tannar'ri make Intimidate checks against speakers with a +4 Circumstance bonus. ("He who gazes into the Abyss..." and all that.) However, against other mortals or Evil outsiders, Intimidate checks made in Abyssal confer a +2 Circumstance bonus to the speaker.
• Sylvan is vague and euphamistic, and more full of indecent proposals than a typical American's perception of French. Intimidate checks are impossible. Listeners attempting Sense Motive checks suffer a -4 Circumstance penalty, and speakers gain a +2 Circumstance bonus to any seduction attempts conducted in Sylvan.
• Draconic is the language of Arcane magic. So, +2 to Spellcraft checks vs. Wizards, and +2 to Knowlege (Arcana) dealing with things written in Draconic. Also, speakers of Draconic know exactly how weak the language's creators consider humanoids, and therefore gain a +2 Circumstance bonus to Sense Motive checks against dragons, but take a -2 penalty to Will saves vs. a dragon's Frightful Presence.
• Druidic oaths are as binding as Infernal ones, but the agency enforcing the oath is far more benign. It is a pity that the language is not more wide spread. However, this means that Druids can always trust each other's word, and this lets them to function together more cohesively than their loose organization would seem to allow.
• The Elemental Tongues are said to be heard in the dreams of Elemental Princes, so secrets should not be told in Aquan, Auran, Ignan or Terran if there is a large quantity of the corresponding element nearby.