Okay, lets get a few things staight . . .
Modern linguistic theory _does not_ claim that all languages are based on one
original 'mantongue' type speak (i.e., from where humans originated) -- just try
and find some real links between Indo-European and any Amerindian language, and
you'll find that hard to maintain. It's difficult!
IMO The point is, language in Gloranthat is _MYTHICAL_, not _evolutionary_ like
the real world, where more than one language may have evoilved for different
reasons. Okay, so in Glorantha it's evolutionary depending on race/religion as
far as humans are concerned, but that _doesn't_ mean human language is based on
a historical progenitor -- it's based on a mythical progenitor.
For instance, 'mantongue' for Hsunchen may mean something completely different
to 'mantongue' to an Orlanthi -- it may mean the original mortal tongue to one,
and the speech of humans to the other . . . i.e., the myth facts of history vary
depending on the myth you believe -- kind of like real life -- religion (Bebel,
or eveolution). But Mantongue still exists, whichever you believe.
So, for the Orlanthi, Mantongue could be the parts of language comon to all
Gloranthans (as some parts of all European languages are based on
Indo-european).
To a Hsunchen, Mantongue could be the original language spoken by all mortals
(with the man rune).
For a Brithini Mantongue could be 'his' i.e., the 'true, pure' language,
uncorrupted by heresy.
For a Praxian, Mantongue could be the language which all tribes have in comman
- -- even the ones not in the pact.
For a Godlearner, mantongue could be a contruct that describes the original
speech, as spoken by (the construct) Grandfather Mortal to his children, but
which has changed since the dawn due to the geographical seperation of his
children.
Oh, by the way -- this is my first go, and it's all _my opinion_.