There is no special trick to it. You just need to know the types of questions asked in JNU entrance exam.
Learn about culture, art, history, literature of the countries where these languages are spoken.
First thing you need to decide is the languages you are trying to get into. After that you need to prepare accordingly.
You can select maximum of three languages from below groups(called cluster).
Japanese, Chinese, Korean
Spanish, french, Russian, German
Arabic, Persian, Pushto
You can select three languages(maximum two from 1 group).
Suppose one of the languages you selected is Japanese. Then you need to learn about Japanese culture, art, literature, music, drama, script, etc. And same goes with other languages.
Generally there are more questions related to Japan and China if you select any languages from that group(cluster.)
Manorama year book is really helpful. Learn ‘Who is who’ segment from this book for sure.
You also need to work on English. Generally they ask article, preposition, verb.
You should be updated with the recent things as there will be a paragraph for any recent issue. for eg last year they asked to write about Demonetization.
Feel free to ask me in the comment box if you are having any further doubts
hope you find it helpful