As Mr. Anoop told urdu was one of the main stream language. at the time of our independence urdu was one of the languages to spread the word, even Pandit Nehru and leaders knew urdu. So as our Actors and directors, there will be hardly few films which was made purely hindi, the songs in the hindi films and most of the dialogues were written and are writting in urdu only,
Actors who can read urdu
Raj kapoor and his brothers
Rajendra kumar and so on
they all know urdu thats why
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In earlier days in India, Urdu was one of the main stream language like Hindi. Our PM Mr MM Singh also reads the scripts in Urdu. Especially people from UP, Punjab and many parts in North India (part which is now in Pakistan) people reads in Urdu. Thats why not only films, but posters, magazines, news papers, sign boards, etc were also written in Urdu. This trend is still adopted in Films, while others it has gone and now very rare people can read Urdu (Old people) and young generation is forgetting the importance of Urdu language.