Would it be good to learn simple phrases in languages like Pashto or Arabic or Korean before joining the Army?

I have lived in Germany, Afghanistan, Korea, been to plenty more, and there is no way I can learn all of those languages. What is important, learn the culture. The more you learn about the culture the easier everything else will be. I learned about the culture of the Philippines, don't really speak Tagalog, and this month have been happily married for eight years because I learned my wife's culture (and liked it) long before we married.

It's good anytime, but especially when you might find yourself living and working among a foreign people. One of the more interesting things I learned while working with the Japanese (I consulted with almost all the Japanese automakers during my civilian career) is that they hate to hear Americans (or anyone else) butchering their language. They prefer to butcher ours to communicate. Although I had studied Japanese, it was useful to pretend ignorance. I could catch a phrase hear and there to decipher what they were saying behind my back. And, I learned how to bow correctly. They may have thought of me as a barbarian but at least a polite barbarian...

Can’t hurt! Like your motivation to learn.