These are the critical languages as cited by the U.S. DoS/DoD and I imagine speaking any one of them fluently would have a more profound impact on an application than a more common second language (Spanish, French, German, etc.):
Albanian, African Languages (all), Akan/Twi, Amharic, Arabic (all dialects), Armenian, Azerbaijani, Bahasa, Bambara, Belarusian, Bengali, Bosnian
Bulgarian, Cambodian, Cantonese, Croatian, Czech, Gan, Georgian, Haitian, Hausa, Hebrew, Hindi, Hungarian, Japanese, Javanese, Kanarese Kazakh, Khmer, Korean, Kurdish, Kyrgyz, Lingala, Macedonian, Malay, Malayalam, Mandarin, Moldovan, Pashto, Persian, Polish, Portuguese, Punjabi, Romanian, Russian, Serbian, Sinhala, Slovak, Slovenian, Swahili, Tagalog, Tajik, Tamil, Telegu, Thai, Turkmen, Turkish, Uighur, Ukrainian, Urdu, Uzbek, Vietnamese, Wolof, Yoruba, and Zulu.