The languages of all Gloranthan mortals developed from a single source, which the God Learners called “Mortal Speech”.
Mortal Speech
There is an ur-language that is the source of all mortal languages in Glorantha. Let’s call that Mortal Speech. This was the language that Grandfather and Grandmother Mortal taught their children to speak with each other. Thus, every mortal language – be it human, dark troll, dwarf, elf, or even menfolk – has certain shared vocalizations in common. All mortal races possess at least the potentiality for communication.
Now that language was first taught over 60,000 years ago (at least according to the Dara Happens, whose chronology I am not willing to dispute). Since then, the mortal races have spread across Glorantha and countless generations have passed. The events of the Gods War separated many mortal races by water, land, and mountain. The division of Mortal Speech into many mutually unintelligible tongues was inevitable.
Runic Speech
The gods of the Celestial Court communicated using Magic, but their children and burtae used runic speech to communicate with others. In the God Time, many of the descendants of Grandmother and Grandmother Mortal mated with the gods, who taught them their eternal and unchanging runic speech.
Darktongue: Creatures of Darkness, such as bats, dehori, Underworld demons, and their ilk, speak Darktongue.
Earthtongue: The speech of the goddesses of Earth, snakes and other reptiles, and other Earth-related creatures.
Firespeech: The celestial language of the Sky World, of fires, and birds.
Seatongue: The language of waves, of rivers, and other the watery depths.
Stormtongue: The language of winds, thunder, rain showers, and mammals.
There is also Spiritspeech, the means of directly communicating with spirits.
The various mortal languages have been strongly influenced by one or more of these runic tongues. For example, the Dara Happen language is strongly influenced by Firespeech, while the Theyalan languages are influenced by Earthtongue, Stormspeech or both. Praxian borrows elements from Spiritspeech, Stormtongue, and Earthspeech! Kralorelan and the other Eastern languages are strongly influenced by Firespeech and Seatongue. The Brithini tongue is strongly influenced by Seatongue and Stormtongue, while the Old Seshnegi and Jrusteli tongue add strong Earthtongue influences. The Middle Sea Empire discovered that Esrolian and Doraddi, two mutually unintelligible languages separated by the Inner Sea and with no historical contact and little mythological connection, share a small number of words derived from Earthtongue.
The more recent language called “New Pelorian” has altered Dara Happan with many words and concepts derived from Lunar sources. It is as yet unknown whether it shares any words with the Blue Moon worshiping peoples of Pamaltela.
Elder Races speech
The Elder Races, being more closely bound to their ancestor gods than are humans, have languages even more strongly influenced by runic speech. Aldryrami is the language of the woods; the different forests of the world have different accents but no different dialects. The dark trolls are so closely bound to the Darkness that they speak the same Darktongue as do their dehori ancestors.
Mostali is unusual in that it does not appear to originate from Mortal Speech and does not have elemental associations.
Draconic also predates Mortal Speech and is extremely different for other races to learn. The Theyalan language spoken in Dragon Pass by the Empire of the Wyrms Friends was strongly influenced by Draconic. Although that language is now nearly-extinct, it is still spoken by the population of Pavis-Within-The-Walls and used in ancient religious ceremonies by the inhabitants of Adari.