The top 10 that seem to be the most important right now for national security jobs:
Arabic: North Africa/Middle East
Chinese/Mandarin
French: West Africa (is often coupled down the line with a more local language like Wolof or Bambara)
Hindi-Urdu
Malay/Bahasa/Indonesian
Persian/Farsi/Tajik
Punjabi
Russian: Central Asia (Russian is a common lingua franca in Central Asia where national security is also a major concern)
Spanish: Certainly if you want to work domestically
Turkish
The State Department gives bonus points to prospective employees who speak:
http://careers.state.gov/officer/selection-process#nogo with proficiency needed to garner the points here
Bonus point languages: http://careers.state.gov/uploads/23/86/2386f5de7f14369e5231db272ccfe423/Language-Points-2013.pdf
Super Critical Needs Languages: Arabic (0.5) the most important with Chinese, Hindi, Korean, Pashto, Persian, and Urdu garnering 0.4
So Turkish 0.17 < Persian 0.4 (at least as far as the government's needs are concerned). If the two finalists are Persian and Turkish, go for Persian if you are eyeing government national security work.