If you were a recruiter, which secondary language of an individual is more preferred to enlist?

Arabic, by far.
I would point out that what is important in the military is the ability to read, understand, and speak the language. A semester's worth wouldn't get you very far.

Many Army billets offer additional training and a few offer Foreign Language Proficiency Bonuses. If you can test out at a high enough evaluation on the DLPT, it is well worth your while.

I hope you're asking so you can decide what to do for after college because, as stated, one or two classes in a language won't get you anything when you enlist in the military. I found a Strategic Language List released in 2015 for the Army.
"The SLL is comprised of three language groupings: Immediate, Emerging and Enduring."
"The Immediate identifies languages immediately needed to meet urgent demands that are eligible for the highest FLPB tier and also authorizes Army Tuition Assistance for language training for the specific language. These languages are as follows: Arabic-Yemeni, Baluchi, Pushto-Afghan (Pushtu), Persian-Afghan (Dari), Persian-Iranian (Farsi), Somali, and Urdu."
"Emerging identifies languages for anticipated future requirements and are eligible for FLPB. Army Tuition Assistance is NOT authorized for language training for these following languages: Acholi, Amharic, Arabic Levantine, Azerbaijani, Bengali, Burmese, Hindi, Kirghiz, Punjabi (Western), Tadjik, and Uzbek.
Enduring identifies languages that represent long-term needs and are eligible for FLPB. Army Tuition Assistance is NOT authorized for language training for these following languages: Algerian, Egyptian, Maghrebi, Gulf, Sudanese, Hausa, Igbo, Swahili, Yoruba, Georgian, Hebrew, Japanese, Romanian, Serbo-Croatian, Indonesian, Javanese, Cebuano, Maguindanao, Maranao, Tausug, Yakan, Malay, Thai, Turkish, Turkmen, Kurmanji, Sorani, Ukrainian, and Vietnamese."

Any foreign language that is spoken fluently is a bonus. wouldnt matter to me what the language is