Morse code

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.

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

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).

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