Spanish, because in the area I live in we have a lot of mexicans. I always feel that they cut us down, they looks at us, say something, and then start laughing. Also I've always found that language interesting because complexity.
With a wish to speak with many people: I would prefer a common language with which i can communicate to many different people who also know that common language but has got different language as their native language.
Oldest language: Or else we can choose oldest languages that are still in use. (speak, write, learn) In order to explore the literature or history. This would be interesting if we go deep to analyse how letters are formed and how it changed.
To impress a girl that we like: If we like a girl speaks some other language. Then we can learn that language and speak with her. She will be correcting mistakes in what you speak and you have got more opportunity to learn language as well as to talk with her.
Plan to Visit a place: If we visit a place as a tourist, A little knowledge about that language helps us to survive their for some days. It would also be interesting as we communicate with their native language.
Latin. It's the basis of a lot of other languages (Spainish, German, other so-called Romance (Roman) languages. Also, it's extremely logical structure sharpens your ability to reason and think critically, two often neglected skills in our country.