Python

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.

French and Arabic are the second and third most spoken language in number of countries. Then there is the obvious Mandarin which is spoken in most of China with around a billion locutors

Depends on the route you want to take while traveling. For example, if you want to circumnavigate in a sailboat through the tropics, French is a great choice because France includes a bunch of tropical islands:

French is also widely spoken in Africa, IIRC.

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.

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Mandarain opens up communication with over a billion people

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.

If you're fluent in Spanish you can probably bullshit your way through comprehending most romantic languages so French and Portuguese (the other big colonial languages) are probably out.

Maybe Arabic for the fact that while it isn't a dominant language in most countries there are fluent communities in all sorts of parts of the world.

Alternatively, Hindi/Mandarin for the sheer number of people it'd let you communicate with.

I'd say French or Portuguese

You say “travel the world,” but where realistically will you travel? Westerners tend to travel almost exclusively to other imperial core countries and to popular tourist spots in the periphery that cater to imperial core tourists.

I'm gonna second French and Arabic for sheer amount of speakers (native or as secondary language), as well as geographic variety.

Other than that, I would've said that Russian would serve you well in the post-soviet sphere of influence, but that changed recently for obvious reasons. You very likely don't want to travel to Russia, and her neighbors don't look to kindly on Russian either, now. Will still do in a pinch

okay, and one joke answer: Japanese, you'll find weebs to talk to in every country

The Harappan language—then you could decipher the script of the Indus Valley civilization.

I've gained a lot in my life learning Urdu and Hindi and encourage more people to learn. They are essentially the same language with different scripts, but unlike Arabic or Chinese they are Indo European languages and are much easier to learn. I grew up speaking farsi so maybe it was easier for me but if you work in IT knowing Hindi is indispensable.

It depends on your goals, and what cultures are you interested in.

I'm also fluent in Spanish and English (native Spanish though), and my third language of choice has always been between Chinese or Japanese. I like east asian culture, technology and many of their cultural exports. So it was an easy choice for me (easy the choice, languages... not so easy).

Mandarin

I mean, most of the world's population lives in China. So Mandarin would be a good candidate.

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'm sooooo close with French. I just need to buckle down for like a couple weeks and I'll be fluent I think. I've always heard that's a good one.

I dunno about useful but Japanese is beautiful and I want to learn it

If you don't know where you're going: French is probably the most wide spread after that.