Steve is right that language ability does not automatically equate to cross-cultural operational capacity. There are folk who can operate cross culturally without knowing the language well, semi-fluently, or even at all. There are also folk who are excellent linguists who can't for the life of them empathize cross-cultuarally.

That said, language is a key part of any culture. One who is without the language of that culture operates at a significant handicap. A book that comes as close as any I've seen to demonstrating the relationship between language and culture from the perspective of a third party player in both insurgency and counterinsurgency is Bill Meara's CONTRA CROSS (discussed in this forum).