Depending on your other personal goals, I would recommend that you learn the language that you are most likely to use frequently. This will reinforce both your skill level and your motivation. 
It can be best to start with either the language of a neighbouring nation or the language of a nation that you are most likely going to trade with. 

For the United States of America, these languages would include French, Spanish and Mandarin. 

Alternatively, suppose you have a lot of free time and want to do something unique. In that case, you can learn a rare or ancient language that will enable you to conduct historical or archaeological studies—for example, Latin or old English. 

If you want something even more exotic, you may want to study one of the languages from the `UNESCO list of endangered languages`. See 

```
https://www.theguardian.com/news/datablog/2011/apr/15/language-extinct-endangered
```

For example, the language "Vafsi" is classified as definitely endangered, with approximately 18'000 speakers currently active. 

I hope that this helps. Please let me know more about yourself, and I might be able to provide a recommendation that is more appropriate for you.
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