What languages would you need to speak to communicate with the majority of humans on Earth?

[Zero]

Ebonics

[Dauphin]

The language of money.

[ColdPhoenix]

Definitely English, Mandarin, Spanish and Hindi.

Arabic, French, Russian etc don't even come close.

[Lefty Scaevola]

Hindi is very common, but it largely overlaps those with English as a secondary language.

[Tattila the Hun]

I thought sign language would be usefull, but apparently, there are dozens of different ones... And, not that widespread, anyways.

[ColdPhoenix]

Hindi is very common, but it largely overlaps those with English as a secondary language.

To an extent but it's understood throughout India and Pakistan as well as a decent proportion of Bangladesh. Include the Indians etc scattered throughout Africa and Europe and I reckon you're getting close to a quarter of the world population. Estimates on Hindi speakers will be lower as a lot of these people aren't Hindi speakers but instead speak a related language that is close enough to enable communication.