My ongoing belief is that the 100 Stories "locked" their language in.
If the priestess has to learn it word-for-word, and possibly even in the same dialect, then that would be a linguistic constant over the ages.
The entire work was large enough that it would provide a decent language base for them.
Since the priestess is the chief's wife, and head of the ruling class, that's probably the prescribed/enforced language for any village.
With no outside contamination, maybe it could last longer than might be assumed.
Kirisame referenced the 100 Stories as well, so there must be a priestess on Treasure Island, which might explain why they (specifically Jafar, who notices everything) don't detect any differences when they're talking to Kohaku and Ginroko.