Language Divergence on a Lost Colony

I've read several of the questions about language shifting and diffusion here, and it has unfortunately left me more unsure of things than I was before.

Situation:

Colonists on an arid planet speak two major languages, Hindi and American English, there are also spacers who speak English as well.

There is a disaster and around thirty thousand people survive split up into groups all over the planet in groups ranging from a few dozen to about a thousand, with the low hundreds being the average. They are based around oases, springs, and along the coastline of a polar sea.

Due to being cut off by arid and hostile land, most of these groups are fairly isolated from each other for the first five hundred or so years. Only the people on the sea which has the largest population interact and trade in large numbers. After that point trade caravans would spend the next five to six centuries reconnecting the towns and cities together, until even the smallest settlement sees a caravan once a year and major cities have a steady flow of travelers.

The only written documents saved from before the fall are technical manuals, a few scrapes of books, and what the survivors write down.

Realistically, after 1500 years how much would the language have diverged among the settlements.

Edit

Would the various towns and cities be able to talk to each other easily, would they speak a creole that everyone can pick up in a few hours, or would they have totally different languages?

Edit 2:

English was the language for administration purposes. Hindi was more for cultural purposes amongst half the colonists.

Edit 3

The initial tech level after the fall is limited. Due to loss of satellites, and most infrastructure, almost all radios are handheld and only really of use within short distances of the camps. If conditions are just right some radio communication can occur several hundred kilometers away, but this is beyond the ability of most survivors.

Communication is largely by foot or short range planes and ground cars, with no way to maintain them, these fail within a decade.

After this point people stay in touch via camel, horse or wagons pulled by animals. They know how to do everything they need, but don't have the capabilities, by the time they start getting the capability most of the knowledge is gone.