English?

This goes for pretty much all contemporary Bollywood films. There are a lot of reasons why English is a popular language in India, not the least of which is the fact that India was a British colony until they were granted independence in the 1950s.

Since then, a large Indian diaspora in English speaking countries (mostly Britain), and increased Western influence in India has caused an increase in English dialogue and song lyrics. English phrases are also a sort of popular slang among youth culture, as well as the adoption of more "modern" dress.

Simply put, mass media culture and the presence of Britain for well over 100 years has made the English language an Indian mainstay. Under harsh British rule, many Indians were forced by circumstances to at least learn the English language. Some Indians were given positions of power by the British to administrate the crown's control of numerous geographic areas. Many were extremely corrupt and harshly enforced the crown's rules. What we are seeing now in India are the generations that followed after British rule ended. Even still, the influence of Britain and the English language is a fixture of Indian culture now.

Many Hindi films have addressed this conflict of Western influence on traditional Hindu values as a central theme of the drama, including the highest-grossing Hindi film of all time, Dilwale Dulhania Le Jayenge (1995). DDLJ, like a growing number of Bollywood films from the early 90's on began to address the Indian diaspora very directly, featuring Hindi characters living in English-speaking countries.

So.. the speaking of English in Hindi films is somewhat of a recent phenomenon, but English has been around in India for longer than you can imagine. Read up on India & the British East India Company if you are interested:

http://en.wikipedia.org/wiki/Honourable_East_India_Company
http://en.wikipedia.org/wiki/India