What is the most common language of India?

You are right, not everyone speaks Hindi fluently (esp. the ones in the south of India), but it is very useful to know the language (esp. in northern areas). English, yes, but again, it depends on the populace you are speaking to. Many rural folks might not know english. There is the prevelence of the local language now which differs from state to state. They are very different from each other. Eg. in Mumbai, it helps to know Marathi, which is their local language. Or telugu, a southern language, in Andhra Pradesh state. I'd be prepared with hindi, english ofcourse, and some knowledge of the local language of the place am visiting. But overall, I'd say it's fairly easy to get by with english and hindi.

Hindi. But people who might not know any Hindi at all might know English and use that as a "bridge language". (Only 40 million people in India with its population of 1 billion people use English as a first language.)

*The SIL Ethnologue lists 415 living languages for India

Speakers Percentage Speakers Percentage Speakers

1 Hindi[5] 422,048,642 41.03% 329,518,087 39.29% 366 M

2 Bengali 83,369,769 8.11% 69,595,738 8.30% 207 M

3 Telugu 74,002,856 7.19% 66,017,615 7.87% 69.7 M

I'm actually not sure whether its English or Hindi at this point. The problem you were having was likely due to the fact that there are a massive number of dialects, some of which are so different they're really not cross-compatible.