English. It's the language of international trade. second: Mandarin chinese: China is a growing economic power and the most populated country in the world. It will, without doubt, be handy.

I think it depends on your future plans. In aviation, it’s a must knowing English as a pilot or flight crew member, or control tower person because all air traffic communications are in English worldwide. And that makes sense. You need one basic language for air safety on an international level at least. I know French speakers tried really hard for years to make French the lingua Franca, but it never worked. But, if you’re in a non-English speaking country and don’t plan international travel much, or at least know the language of the country you’re traveling to, you don’t need English. Unfortunately, the universality of English has made native speakers of English in the US, at least, incredibly lazy about learning foreign languages. Part of it is the universality of English, on the one hand, and the size of the US on the other. It’s really a shame.

Fasu, Western Apache, or Nambikwaran. All three have five-term evidentiality: whenever you make a statement, you have to cite your source: "I saw this myself" vs "I heard this from a reliable source" vs "everybody knows that this is true" vs "by saying this, I make it true". If you learn one of these languages, your mind will become far better-organized than it is now. What could be more useful than that?

No such thing. It depends on what you want to do/where you want to live. People say that Chinese is useful, and it is...if you want to live or do business in China. But if you want to live in, say, Hungary, Chinese will be absolutely useless to you. That said, if you have no specific plans...either pick the one that interests you, or pick one with a lot of native speakers. French or Spanish, maybe. Or Portuguese. Or Russian. I wouldn’t recommend Chinese to someone who isn’t sincerely interested in it (it’s a lot of work) but it is also a good useful choice.

good point, however I mean for practical use, giving you the maximum access to the world