After the collapse of our Ancient Civilization, communications go back to the condition of the Middle Ages or worse, where nobody ever hears anyone speak who was born or raised more than maybe 50 "miles" away.  Dialects drift rapidly in both Simelan and the surrounding Gen languages, and languages continue to evolve. From: Jacqueline Lichtenberg 2013

In the Sime~Gen Novels the main languages referred to are Simelan and Genlan, which is what Kreg, from Ambrov Keon, calls English.

Listen to some of the Simelan words spoken by the author here -

Linguists studying the languages in use at the time of the Modern Tecton can trace all the Gen languages back to their various ancestral roots and thus trace migrations of populations around the globe.  The historical record of language for the Gen cultures works exactly as it works today for our Ancient languages.  Simelan though is very different, and very puzzling.  It appears suddenly, has no clear ancestral roots in any Gen language, and has vocabulary and syntax that are not cognate to any Gen language.  Simelan handles space and time in unique ways suitable only to Sime Perceptions

The immense, linguistic science defying puzzle of the origin of Simelan is not solved even during the space age.  It is simply accepted as a fact of life by most, and in various ages used as "proof" of the origin of the Sime/Gen mutation split (proof for whichever theory holds sway at that time in history.) 

In First Year, Simes pick up Simelan effortlessly just on hearing it spoken in conjunction with nageric field-shifts, and after just a bit of practice, they "get it" and begin inventing and deriving expressions.  Oddly (and in defiance of the rules of language as they are known to Ancients), once clued in all Simes derive extensions of the basic field-descriptive language that are intelligible to other Simes.  If exposed to Simelan usage by a group of Simes during First Year, the new Sime will quickly master Simelan with full intelligibility to other Simes elsewhere in the world -- but of course with wildly varying accents which are difficult to decipher. 

The result is a great variety of Gen languages distributed around the Earth just about as we see today, and a single language used by Simes almost uniformly around the world, wherever contact with the spoken Simelan spreads. 

After centuries, Simelan "borrows" and acquires words, expressions, phrases and ways of thinking from the surrounding local Genlan, so with time Simelan develops geographical dialects and variations that are hard for foreign Simes to understand.  By the time Mass Media girdles the globe with live-spoken language, variations even out.  By the space era, most Simes can pretty much understand most other Simes -- if they listen closely. 

Simes can and do learn various Gen languages, but Gens can not master Simelan (because the Gen brain does not develop the synapses required to perceive space and time the way a Sime does).  Gens speak Simelan in a simple, everyday manner as would a child raised by Simes.  A child can speak Simelan quite well for most purposes, and even guess at nageric and ambient based expressions, so adult Gens can learn to discuss almost anything in Simelan.  Polite Simes don't point out their errors.