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I'm in England, and English is the only language I speak fluently. I know a smattering of French, Arabic and German, all of which occasionally come in handy as sometimes work with people for whom one of those languages in their mother tongue.
Every so often I think "it'd be worth me learning to speak better French", but then a few months later projects/priorities change and I find myself thinking the same about Arabic, or whatever. If I was working on projects for customers in Spain or Mexico, I'd be thinking about learning Spanish.
I don't think there's a definitive answer to this. I have found though that even knowing how to greet someone in their own language can do a lot to endear you to them, as it shows you're making some sort of effort
Also, I often find, because the English are notoriously bad at learning other languages, that foreigners will naturally converse with me in English. In any context, it's really good to be aware of any cultural-specific things which you need to be aware of (e.g. certain hand-gestures considered commonplace in one culture are offensive in others, which pocket you place a business card in is really important in Japan, etc)