As I've mentioned before, the various human and demi-humans of Mor-Thir speak many languages. However, I'd like to discuss where these languages came from, how they came to be what they are now, and perhaps most importantly, what languages exist.
The human tongue, usually known as Common, is descended from Nobiran, Dwarf, True Elf, and various other languages; humans are adaptable, if nothing else, and have molded their languages over the eons to fit the cultures around them. Common sounds quite similar to English, with all its various dialects. Men of Mareten still use Nobiran as a ceremonial language; it sounds quite similar to Latin. It was the original language of Men, when they landed on the eastern shore.
In the south, the desert people speak Zaharan, a language descended from the lizardmen's tongue. The language resembles Turkish. Men and lizardmen have shared the language, and adapted to each other; though there are regional dialects, almost everyone in the desert speaks some form of Zaharan.
In the Wild Plains, the language of men is descended from Nobiran, though time and distance have changed it into a language of its own. It is similar to Spanish. The language originated in the kingdom of Mejasta, and spread north and east through the Wild Plains. Today, most tribes living in the plains speak Mejastan.
Across Dragens Rygg is the Gwyllt Jungle; though little is known of the interior, the natives there speak a language with similar roots to Mejastan, though it is obviously a different language. The only known city that speaks the language of the jungle is Sao Agua, though there are many tribes that live in the jungle, of men and beastmen. The language is known as Selva, and is quite similar to Portuguese. Few speak it - or have even heard of it - outside the Jungle.
Both the Northern Dwarves and the Deep Dwarves speak a language not unlike Norwegian, though their time apart may have separated the languages somewhat. The language is simply known as Dwarf, though the Hill Dwarves, having spent much time with Men, speak Common.
Long ago, the True Elves spoke a language similar to Scots Gaelic, but the few that remain have spent so much time with Man that their language has evolved past understanding. The language of Elves spoken today bears only a superficial resemblance to its roots. However, the gnomes also spoke the language of elves, and through song and story have kept the language much nearer the original.
The Fae speak a lilting language similar to Welsh, beautiful and complex. All faerie-kind, be they from the Winter or Summer Court, speak Fae, and it has gone unmodified for as long as it has been a language, for the faeries are very strict when it comes to the meaning of words, and every word means precisely what they intend. No human or demi-human could hope to grasp the subtle nuances of the language, and is likely to end up making a promise they cannot keep; it is a good idea to speak very carefully around the Fae.
The beastmen each speak their own dialects, usually unrecognizable as anything human or demi-human, including but not limited to: Orc, Hobgoblin, Goblin, Gnoll, Kobold, Neanderthal, Troglodyte, Swamp Lizardman, Draconic, . As a note, the desert lizardmen speak Zaharan; the lizardmen of the swamps have been away so long, the language has devolved into an unrecognizable form. Bugbears speak Hobgoblin and Goblin; Ogres speak Orc, usually. Dragons all share draconic, though they often speak other languages as well.
Finally, there are a number of languages invented for one reason or another; some, like sign language, are used when normal language is difficult, such as by the deaf, while diving, or when moving stealthily. Others are codes, such as Thieves's Cant, a coded language similar to Cockney rhyming slang. Understanding the base language - say, Common - is not enough to understand the encoded message. For most invented languages, the words, phrases, or motions are usually highly localized. A Mareten sign language for the deaf would likely only be understood by the deaf in larger cities, while a Thieves' Cant from the west side of Riverbend may not be understood by a thief from Slohaven - or even a thief from the east side! Other signs, cants, or codes are likely limited to a specific tribe or organization.
Thus, the major languages of Mor-Thir are Common, Mejastan, Zaharan, Nobiran (or Church Nobiran), Selva, Dwarf, Elf, Gnome (or True Elf, or Old Elf), Faerie, Orc, Hobgoblin, Goblin, Gnoll, Kobold, Neanderthal, Troglodyte, Swamp-lizard, and Draconic.