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That really depends on what you want to do with it. If you're looking to get ahead career-wise, English (if you haven't already nailed it), Spanish, or Mandarin are solid options. If it's just for fun and you want to explore something culturally new, I'd say: pick a language whose culture actually appeals to you - that way you'll stay motivated while learning. What interests you most anyway?
Don't just go for the "biggest ROI" - that's the wrong approach. Sure, Mandarin or Spanish sound attractive, but if you don't have a real connection to the language, you'll quit after two months. I see that all the time with people who only learn because it's "supposed to be" worthwhile.
Better: Think about what you actually need the language for or where it's fun for you. Do you like films, series, books in a certain language? Do you have family or friends you want to communicate with? Or do you travel somewhere regularly? Those are the real reasons that keep you going. Japanese might be "less useful" than French, but if you love anime, you'll learn it with way more motivation.
One more thing: don't underestimate how long "learning properly" takes. Pick a language you're willing to stick with regularly for at least a year, not something that sounds cool for three weeks. Languages close to your current language (if you speak German: Dutch, Swedish, Norwegian) are faster, but they need less motivation - because it's just easier.
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