What is the national language of India?

What is the national language of India?
Which language should learn to visit India?

Referring at various source, I could not find single national language of India. Although all the stated have their own local language, however majority of the states have accepted 'Hindi' as national language but because of few states, Hindi is not the officially declared as national language yet.
Dr.Ambedkar, the architect of Indian constitution wanted Sanskrit as the national language of India. However, India didn't choose any national language because of different opinion of few states.

But, in most of the places of India or majority people of India understand Hindi. So, for the tourist, or the person who want to visit India, person needs to choose the language based on the places that he/she wanted to visit. If the person is going to visit all metropolitan cities then English and Hindi would be best option, because either of these language will be definitely spoken.