If I could only learn one additional language, and I wanted to travel the world, what language would serve me best other than English or Spanish?
By the numbers: French or Arabic, as other commenters have mentioned.
But it really, really depends on where in the world you want to travel. If youβre interested in Asia, for example, neither French nor Spanish nor Arabic will help you much (save for some remaining French usage in Vietnam).
A better answer is: figure out where you want to go, then do the math on what to learn.