So around 13,000 years ago, a group of refugees from a planet with Greco-Roman culture and technology were kicked off of their planet by what were essentially space vikings (space travel is done via magic instead of via technology in this universe so don't worry about that). Anyway, after a hundred years of traveling the cosmos, they wound up on this planet called Aurea. With a Mediterranean climate similar to their home planet and an extreme richness in precious metals, the Guants (what the refugees were called) decided to stay. However, the planet was very different from their homeworld in other ways.
Some areas of the planet were also home to native tribes. The far west was home to the Zebru Kingdom (essentially a more urbanized Zulu Kingdom). The far east was home to the Tengol Tribes (several constantly warring Mongol-esque tribes that constantly fought with each other and are expert horse, hippogriff, pegasus, and dakotaraptor riders).
Over time, the Guants formed their own kingdoms, one of which soon grew so powerful that it was able to completely unify the planet (including conquering the native populations) and form the Aurean Kingdom. Back when they lived on Olympia, Guant culture and technology resembled that of Byzantium during the time of the Macedonian Emperors.
Now, in the present, their culture is completely unrecognizable to the remaining Guant population on Olympia. While Guants on Olympia have largely stayed in touch with their old culture, the Aurean Kingdom has become something entirely different. Most importantly, the Aurean language is now completely incomprehensible to the Olympian Guants. While the Olympian Guants speak Byzantine Greek, the Aureans speak their own related language called Aurean (which incorporates many Zebru (isiZulu) and Tengol (Khalka Mongolian) loanwords and is a tonal language, as well as being extremely agglutinative).
Here is my question: Under what circumstances could these linguistic changes I have talked about be plausible? What would the relationship with the conquered tribes have to be? How much of Aurean would it be possible for the Olympian Guants to understand? What are some more differences that could arise from contact with these two languages for so long?