Let’s think about the Neo-Thracian language.
The Point of Divergence is a vision received by a early 4th-century Bessi oracle, telling the Thracians to switch from worship of Dionysus to Pluto and unite against the Christians. The oracle’s charismatic interpreter becomes the chief priest of a new cryptic cult of Pluto, which becomes a secret society of spies and assassins, centered in the Rhodope Mountains.
A hidden nation of Thracian-speaking people thus survives to modern times, pretending to be either Greek or Bulgarian or Turkish and murdering any outsiders who found out the truth. Fun!
So I’m thinking the Neo-Thracian language is spoken overwhelming by second-language learners, with most people only learning it as teenagers when they’re initiated into the Mysteries of Pluto. By the 19th century, the only people who speak the language from birth are hereditary (female) oracles and (male) priest-interpreters. Which means the language should be fairly standardized and isolating, right? What else should I be thinking?