I would look at the amount of writing in each language, how often they are translated into one another, where they are sold, what are the demographics for each, do certain languages cater to certain classes of people. I would also want to look at things like themes and ideas. Is there certain things they write about? If English writing is considered post-colonial because it deals with the issues of getting back to the original culture and de-colonizing, what if native language writing is dealing with the same themes? Is it post-colonial? If it is in a native language, say Hindi, (sorry Tamil, no bias, I swear) and it is effectively completely controlled by solely Indians, can it be considered post-colonial? What literary forms occur in native languages? Indians (and most countries for that matter) often express a desire to resist westernization. How much has western ideas infiltrated native writing? How closely tied to the rest of the western world is English literature? How closely connected to the past literary tradition is literature in native languages? Does Indian writing in English often follow genres and forms usually found or created in native Indian languages?
Basically I would be looking at the literature, not the people. I would need to see what is available for sale, what is in libraries, what people own, what is taught in schools. I guess all of this would be impacted first by finding out prevalences. If there really isn't much of a certain language or something, that would be highly indicative. Information from publishers and authors would be helpful. Even though this is all literature analysis, essentially, I would have to be in India since native Indian language texts really do not make it far beyond the borders of India.
The goal of all of this would be to be able to put together a sort of ethnography? study? report? (I don't know what you would call it) on literature as a whole within India. This would then allow a more even perspective of English literature in India. Through this I would be able to answer questions about whether English is a weird upstart, whether it is a significant literature movement, if it is ,as Salman Rushdie claims, an Indian literary language, and whether or not it meshes more with Indian literature as a whole, or with post-colonial literature.
This topic does have some problems in that this would require the study not only of works in English, but in other languages. And not just one other language, but a bunch of different languages. It would be impossible for me to learn the languages enough to do this sort of thing. I could probably get Hindi where it needs to be in time, possibly Urdu if I had to. (since I essentially just need to figure out the dag blasted alphabet) However I would have to rely of people and/or translators for books in other languages. I am concerned on how possible that would be. This could be too huge of a topic. It might be better to focus on something like English literature versus Hindi literature. I am also not sure if it is possible to look at this from so many different angles at once. It might be necessary considering time and skill constraints to simply look at publishing rates and readership or else just themes and movements or something. I still also have to deal with the sheer mass of each body of literature which each might make a study of even either of them by themselves impossible.