Ceretis once spoke the same PARENT LANGUAGE as Imperium speaks / spoke; (UNKNOWN NAME / Was to be named.) This was a shared universal common language shared by Ceretis, Imperium, and countless other nations in the 'Latin region' and was the 'common tongue' for the overall area and was not confined to the area inside a political border. Think of the parent language as a giant blob on the map that many nations sit within. The language would most likely have been kept rather cohesive by trade routes and other contact between nations of the time.
This language would have been the language of commerce of the era at least in this particular area of Eras. As the language changed people kept in step with the language as it's the common tongue and would have only had variations in phonetics, and some localized slang. Meanwhile in Imperium, that parent language begins to take on new words and grammar rules which spread to the other parent-language speaking nations they're in contact with, which spreads as well to Ceretis.
This turns the parent-language (at least in some areas) into Latin and by the cultural and commerce chain changes the spoken language in Ceretis from PARENT LANGUAGE to the new language which becomes LATIN. Having contentious relations (politically, ideologically, etc) with Imperium, the people of Ceretis begin to diverge from this new (Latin) language born from the Imperium. Over time this disharmony with the Imperium forms Ceretian Latin, the seed for the future French language.
Then later in 0 AD, August dissidents leave after Paxcism is institutionalized, and some go to ABC while a larger group arrives in Esroniet and speaks August Latin while Ceretis speaks Ceretian Latin. Then, the group splits- some sail to Lanceria, some migrate to modern Zyvun, while others stay in Esroniet. Lanceria, Ceretis, and Esroniet all trade, and Ceretian Latin further develops into French, which becomes a popular / common tongue in the area around Ceretis and the major trade partners of Ceretis of that era near Ceretis and across Eras. After Ceretis colonizes Saint Owhela, some Ceretian dissidents even leave and found their own country far from the cultural French nucleus called Valencia.
The language would have most likely have advanced toward the new language (French) by movements in Ceretis and alterations and changes in the nations there was contact with. Today, many nations speak French especially those nations that were part of that trade network when the language formed and helped in the formation and transition of Ceretian Latin to French.
Sorry.