In the setting I'm constructing, humans migrated in a single major wave to a pair of large continents within the last thousand years or so (the exact value is partly dependent on the answer to this question). While they initially settled in the coastal regions (as one might expect), over time, the descendants of that original culture have claimed, and sometimes conquered, lands spanning an area roughly equivalent to Europe in size. In the current iteration, cities and nations developed and crumbled and grew again in that time, ultimately resulting in the current, pre-industrial state of the region.
So, the question I've hopefully led up to is this: Supposing that all initial immigrants to this "New World" spoke the same language and shared a largely-similar culture, how long would it take for various subgroups to develop mutually unintelligible languages?
- The region of heaviest colonization and city development is comparable to the Mediterranean in size and shape, so relatively constant sea-based trade and immigration is possible among the major city-states and nations.
- Assume a starting population of approximately 150-200 thousand migrants, with no further migration from the "Old World" (at least nothing appreciable).
- At the onset, the culture has a largely-shared religion, one which has had many aspects more-or-less proven empirically.
- Technology level begins in an approximate Age of Sail, though, the process of this migration is precipitated by a major calamity, so quite a bit of knowledge is lost in transition.
- Magic is not well-suited for long-distance communication of any sort.
- Natives are something of a tricky matter, as there are a couple of categories. Where humans are concerned, the northern continent has a number of tribes (mostly hunter-gatherer, with a handful of legitimate settlements), most of which are north of a number of geographic boundaries that would prove difficult for early exploration. The most accessible regions of the southern continent have a culture of nomads who are relatively cohesive. In both cases, though, all humans are outnumbered significantly by another species/race who only tangentially interact with the other natives, but are capable of verbal communication. At most, there are only a few groups who could be perceived as having anything beyond bronze age technology.
This is my first question here, so please let me know if I need to format this differently, or need to add any further information. Thank y'all!