Is the most useful language to learn other than English?

Im thinking of learning Spanish like Spain Spanish because I think it sounded cool also because there are many Spanish speakers on the world but also Chinese Mandarin because of businesses/trades. I can try to learn both but not sure which one I should try first.
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7 mo
I could have sworn I typed "what do you think is the most useful ---" The first four words are gone. I was typing on a bright sunny day outside and I must be tripping.
Is the most useful language to learn other than English?
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