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Whatever language is useful or practical for you. That's really all that matters. If you want to learn one to advance career, it's entirely dependant on your field of work, your potential clients, areas of operation, etc.
If you want to learn a language for private use, again only you know where it would come in handy. Want to travel or move somewhere, want to immerse yourself in a certain culture, want to watch animu? These factors decide what language is good for you.
Don't make the mistake and just learn something because you're told it would be good. Millions of people that were told and forced to learn another language in high school and then forgot all about it once they graduated because they had no application for it can tell, it doesn't work like that
