[[Urdu phrasebook|Urdu]]. There are also other less prominent languages like [[Tulu phrasebook|Tulu]], [[Bhojpuri phrasebook|Bhojpuri]] that are the main spoken language of some places.
'''Hindi''', spoken by 30% of the population, is the primary tongue of the people in Northern India. Many more people speak it as a second language. If you can afford only one phrasebook, pick up the Hindi one, as it will enable you to get by in most of India. The exceptions are some states - [[Tamil Nadu]] and the [[Northeast (India) |Northeast]]. Avoid speaking Hindi in Tamil Nadu, as the language is
often met with some hostility from locals there.
In any case, you are better off picking up as many words of the local language of the place you are going to - people are proud of their culture and language and will appreciate it if an outsider makes an attempt to communicate in it.