Unfortunately I haven't really got the time to try this out right now (but as I think it's an interesting feature I'll be sure to try it out later) but I'll add some stuff about South Asia in general (some of it you may already know and have covered but it doesn't hurt to mention it) :
Persian should be the preferred language of many muslim states and not just the ones in greater Persia. It was the court language of almost any muslim South Asian state in this era. The language and the culture had a status very similar to that of the French language in 18th century Europe. To some extent it was even used in the Hindu courts (they certainly knew how to speak it but they seem to have borrowed more from the actual persianate culture of the Indian Sultanates than the language itself, the Vijayanagara ruler had a Persian style throne room, kept up with happenings in the Timurid empire and among his title counted himself to be "Sultan of Indian Kings").
Dakani/deccani
http://en.wikipedia.org/wiki/Deccani was a lingua franca language of the Deccani states. The real situation was quite complicated in these cosmopolitan states though with different languages used in different situations within and by the state itself. Still, the language should probably be in there.
A situation such as the one in the deccan could really be made a lot more interesting by a mod like this though. The ruling persianate aristocracy was still so new in the area as the game starts, and far less entrenched than the one in northern india,that language did become very important. With a constant influx of persian, afghan and turkic immigrants necessary to make the sultanates function languages became one of the defining differences between the factions at court. The language used in administration and at court played a major part in the conflict between the indigenized dakanis (third or second generation persians/afghans/etc as well as the old local hindu aristocrats) and the gharbians ("westerners", first generation immigrants).
Also for some the Dakani language might even be their native tounge. The sultanates imported lots of Ethiopian slaves, some who would rise to quite high positions (even prime minister/peshwa) and to them the mix of Persian and local languages would have been the only language in which they could communicate.
Not sure how you'd go about covering all that (might be a bit outside the scope of this) but I thought it might be good to have the overview anyway

. In general the closer to 1399 you get the more the persian and dakani languages are used. By the 17th century the sultanates had more and more switched to the local languages for many tasks of the states (but not entirely, when the Marathas formed a state in the area formerly controlled by the Sultanates they actually kept Persian and Dakani as their languages of administration initially).
Urdu should probably also be included though it shouldn't exist at the start of the game (initially the Mughals used Old Chagatai Turkish but mostly they used Persian themselves).