Which language I need to learn ? after English
What language do you want to learn?
If you want to think in terms of practicality, first think about which language would be useful in your life - is there a particular country you want to visit or even live in? Do you have family members who speak another language? Do you want to be able to access a certain linguistic population for business reasons?
The most widely spoken languages, after English, would include Spanish, French and Arabic. Any and all would allow you to speak to millions and millions of people. Mandarin, though not as widely spoken as the others, has the most speakers of any language and could also be useful for business. Hindi is similar in that way.
Whichever language you choose, you should be interested in learning, otherwise trying to reach fluency will be a drag!