What is the most important language to learn now other than English?

It varies from person to person in accordance to their circumstances. Learning a knowledge is beneficial only when you practice it. If you don't get a chance to use it then you forget it. This is something which needs practice to survive. So everyone can analyse his/ her circumstances and select one language that can benefit him/ her.

German, Russian,Chinese,Spanish, Hindi and Arabic.  These are the most spoken languages around the world, it depends on where you want to live or where you are moving to.

In the EU, speaking English and German is an advantage while Hindi or Arabic will not get you far,very few people speak Hindi in the EU. But knowing German is not an advantage in the Arab World or in Latin America. Knowing Spanish is good, but it will not help you if you go to China or Russia.

So it pretty much depends on where you are or where you want to move.