The core problems as an English speaker are both choosing language(s) and actually getting to use the language you learned.
Mandarin, Japanese, Russian, German, French, Spanish, etc are all languages with some value in learning, but which one do I pick, and will I get the opportunity to use them?
If I decide to move overseas one day, then learning their local language would be an easy choice and would your learning efforts would be constantly supported by your new environment.
I could just choose to learn most of the important languages, but that is not only extremely difficult but time consuming. I already have a career and responsibilities.