Some new information on how language will work in MGSV: The Phantom Pain has emerged during Episode 29 of Kojima Station. We have heard before that Snake will come across people speaking various languages, and that will have an effect on the game. As reported by JunkerHQ on Twitter:
“Not only will you not understand soldiers speaking a foreign language if you don’t have an interpreter at Mother Base who speaks their language, you won’t be able to interrogate them either.”
So you will have to find a translator and recruit him. Also, while Snake was fluent in Russian during the time of MGS3, he can no longer speak the language in The Phantom Pain. That part of his brain has been affected by the accident.
Source: Kojima Station Episode 29, JunkerHQ