The number of native speakers for top five major languages as of 2001 are:
Hindi (and dialects) 422 mil
Bengali 83 mil
Telugu 74 mil
Marathi 72 mil
Tamil 61 mil
These are not insignificant populations. If you add in Bengali speakers in Bangladesh, the total hits 189 mil.It's easier to think of India the way you think of EU: an aggregation of peoples and cultures under a political umbrella. As an example, sure many people in germany speak english, but would you really try to seriously gain entry into the german market without localizing into german?