Do you get paid for speaking Mandarin? Is it MOS specific?

Its called FLPP (Foreign Language Proficiency Pay). However, Mandarin is no longer a paying language since 2013. Check the latest memo from Army G2 on FLPP eligible languages. It varies from year to year based on Army's need.

SPC Rodriguez,
The key is to be assigned in a language-coded billet for Mandarin (CM) or to have a waiver if the language is mission-essential for the position. I'm a CM linguist myself and like 1LT Li stated, it fell off the priority list in 2013 putting the billet requirement in effect.

It is not job or MOS specific. Your friend has to pass two of the three areas with a certain level of proficiency (reading, writing, speaking). If he did not pass then he will not get paid. I have a friend that is from China and speaks fluent Mandarin and he barely passed, after taking it twice. If you want to know more about it check the link below. It contains everything you ever wanted to know about pay in the military. You want to go to chapter 19 for this. Good luck, I hope this helps.
http://comptroller.defense.gov/Portals/45/documents/fmr/Volume_07a.pdf