Which languages should one learn?

 The languages you have most use of. The languages you want to learn.

Now, if you want to learn most useful languages, in general...

America is a pretty easy thing, being rather monolingual. You can communicate with almost all, if you speak Spanish, English, French and Portuguese.

To communicate with Africans, add Arabic to the list. These five languages get you through most of Africa.

To communicate with Europeans... most Europeans today manage in English, but it helps if you add Russian and German to your list.

To communicate with Asians... again, most Asians speak English as well (modern, urban Asians at least), but if you add Chinese to your 8 languages, that covers a lot more. Add Hindi/Urdu and Malay to the list, and you cover some more ground.

- English
- Spanish
- French
- Portuguese
- Arabic
- Chinese
- Russian
- German
- Hindi/Urdu
- Malay