Consider a planet initially colonized (1500 years ago) by a group of about 200.000 people.
To prevent ongoing interference/influence of the high-tech society from which they originally came they cut of all contact.
They have elected to "go back" to a more rural/pastoral lifestyle.
No electricity, combustion engines, steam-power, but they are by no means primitive.
They do have printing presses and retain knowledge of science as far as it is applicable to their tech-level. Education (roughly high-school level) is freely available to the entire population and literacy is nearly universal.
People travel widely across the planet. Most nations are located in the coastal regions of large island continents and there is a lot of trade by ship going on between the islands.
Special consideration: After these 1500 years the population has grown to about 25 million people.
Some of those people is a sub-species of man-kind that has a significantly longer life-span. 500 years in stead of the normal 100 years.
These people were already present among the original colonists. They can (and do) interbreed with the normal humans and the children have a 50% chance of being long-lived too. Being long-lived also occurs (about a 1% chance) as a spontaneous mutation in children of otherwise normal parents.
In total the long-lived ones make up about 2-3% of the total population.
These long-lived people are fully integrated into society. Due to their age they gather more experience over time and tend to congregate into important positions in society. Their long-term view of things is seen as an important stabilizing factor upon society. Normal people usually don't begrudge them their long life as they realize that it also has its drawbacks: E.g. to outlive your loved ones.
Initially the entire society spoke the same language.
What would have happened to their language after, say, 1500 years?
Would the language evolve into various regional dialects and from there into separate languages?
Possibly keeping their original language as the lingua franca of scholars, scientists and diplomats? (Just like Latin remained in wide-spread use in Europe until well into the 18th century.)
Would the presence of the long-lived people put a brake on language shift?
Are there other factors that would prevent significant chances to the spoken and written language?
Ideally I'm looking for a plausible reason that enables an anthropologist that visits the planet after 1500 years (and who is reasonably capable in the language of 1500 years ago) to understand the locals and could blend in so he/she is able to travel around incognito.
(With blend in I mean: The locals thinks he/she has a funny accent, but it's close enough to let them think he/she comes from a nation on the other side of the planet.)