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