Diplomatic jobs

I speak English, Spanish and French. I would like to work as Diplomatic, in an international organization or similar jobs.

I do not know which other language to study. Which one would you recommend?

Thanks.

In the US Department of State, they give extra points to applicants who can speak one or more of the following languages:

Arabic
Farsi
Dari
Hindi
Urdu
Mandarin Chinese

Russian or Persian or Chinese or Arabic.