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I don't think there is a right or wrong way to pick a second language. Is there a language you have always wanted to learn? Is there a different language that many people in your area speak and you might want to learn? Are you planning on travelling to another country where it might help you to speak their language? Are you thinking about taking a job where a foreign language would be required?
There are more question you could ask yourself, but it really comes down to what language do you want to learn?