I think its pretty realistic that it only changed once. After all Randland was one homogeneous society with a shared tongue in the Age of Legends and has had no outside interaction with other societies which usually causes a language to shift and alter.

Its also been demonstrated in the books that the common tongue is only the last of many evolutions of the old tongue, Thom mentions High Chant and various other forms of languages throughout the story.

The fact that the seanchan have the same language can be explained by the fact that that continent is controlled by invaders from Randland that already had the New Tongue as their language.

There obviously is a hole here when you consider that the natives of seanchan at the time of the invasion should have been speaking something derivative of the old tongue but that developed separately from the Randland Tongue.

It could be possible that the native language was similar enough to the Randland language because it shared the Old tongue as a root language that by the time of the consolidation the original native language had been subsumed into the dominant randland language in much the same way that French came to dominate England after the Norman invasion because the aristocracy spoke it etc.

Very little has been seen of shara and so it is entirely possible that the majority of the population speaks a different variation on the old tongue with only the traders being able to speak Randlandish (probably very easily because of the shared root language).

It would be very interesting to see someone from "the island of madmen" mentioned in the encyclopedia because apparently they havent made contact with anyone at all since the breaking so they should have an extremely different language.

Rant over