Considering the world power alignment and immediate use potential for Russian or Mandarin, those would be my top suggestions. In terms of post-military life, Mandarin will be key in almost any global business pursuit. I use my Mandarin language minor skills during weekly calls with factories in China. During BCT, I was told the combination of English + Mandarin allows you to speak to more than 95% of the world's population. Today, most of the world population is centered in China and India. The majority of business professionals in India already speak English, so you are presumably already covered there....