The six official languages of the UN are English, Spanish, French, Arabic, Mandarin, and Russian. Hindi (and a lot of other languages) have more native and secondary speakers than Russian, but Chinese and Hindi are relatively geographically concentrated.
It depends on why you want to learn it. While I am on record as rejecting ethnicity as a meaningful concept, if your grandparents are Italian or Japanese or whatever, you might feel like learning that for the family connection. If there's a
For practicality, I would learn French or Arabic. If you can speak English, Spanish, and French or Arabic, you can go almost anywhere in the world, especially outside of southern and eastern Asia, and find someone you can talk to. For enjoyment and learning for the sake of learning, I would go with Greek or a dead(ish) language like Latin.