Lhankor Mhy is the source of all written languages. He took the Runes and used them as symbols for writing. The other gods took that and made use of it. A good scribe, Lhankor Mhy kept the records for the gods, regardless of who they were.
On the Lightbringers Quest, this proved invaluable for the other gods, for Lhankor Mhy knew things hidden from the others. This saved his companions many times.
Lhankor Mhy’s mortal followers were scattered across the world, and scripts changed. For a thousand years, scripts were handed down from father to son, without contact with other sages. Most of these lineages died out. Those that did had changed during the Chaos Age and the Grey Age.
This is why the writing of the Council of World Friends is so different from the ancient writings of Yuthuppa and Raibanth or from the writings of Kralorela or the West. But ultimately all are derived from the Runes, although they are no longer even remotely mutally intelligible.
Two of these lineages are well-known. Around Dragon Pass, there was a small band of scribes that holed up in Nochet and kept the scripts. They sent scribes to aid Heort, and taught their secrets to others who worshiped the Knowing God. They aided the World Council of Friends, and no tribe or settlement was without its scribe. They worshiped the Knowing God by the name of Lhankor Mhy (probably just means something like “Knower of Scripts”), and by this name literacy spread from the edge of Prax to Ralios and Fronela.
Meanwhile in the Redlands a smaller lineage survived among the Starlight Ancestors. They kept records of the sky and its changes, and from that aided their tribes in establishing their rule over the grasslands of Peloria. They worshiped the Knowing God by the name of Buserian (probably just means “Cow(skin) maker”), and their script became used by the rulers of Dara Happa.
Somewhere around 300 both groups began interacting with each other – several generations after Plentonius. Both learned to cooperate during the God Project, and both acknowledged that they both received their secrets from the Knowing God. Perhaps they would have eventually had an academic conference and adopted a unified script, but with the Broken Council that never happened.
Now you can compare this to the Dara Happan sacred script (which became fossilised in place around 250 ST):
Thus in Glorantha we have our Tower of Babel parallel with the shattering of the Spike. After that, scripts and languages were scattered across the world. They changed and no longer were mutually intelligible. But through the Knowing God we can restore that unity – and that’s why the Translate spell rocks!
See also: