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 you're 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.
Kinda surprised that no one has mentioned sign language. I feel that it's use cases expand outside the original intent, especially if other people in your circle understand it.
Since you're interested in traveling, I would say french. It is spoken in a very wide variety of countries all over the world, you have a lot of resources to learn it and it's related to the other languages to know. Other languages might have more speakers, but if youre just interested in the visiting a lot of wholly different places, it is probably the best one.
i don't think people understand how spanish's utitity matters in the future word in light of the big role it played in the this election and i think that's manifested in vagarious forms of ai like llm's and face detection.
in the end, the fact that the global south is too brown and speaks so "humanly" holds face detection and llm ai's at bay as we descend from this election into the world orwell described in 1984; financed by popularly beloved american oligarchs; and enabled by a two party system through manufactured consent and crowd/social engineering.
and to our new big brother overloads i say this: good morning; i'm pleased to see you again and what would it be my pleasure to do for you today? lol
i saw that and i shared in the hopes of creating discourse that's both relevant to the "interesting" times we've been living in these last few days and add a new perspective for everyone to help encourage engaging with you through your post.