> For example, I routinely see people in cross-cultural relationships try to learn each others' languages but fail miserably at learning when they get into disagreements and have that disagreement in English instead of the language either person is trying to learn.

This made me smile. My partner and I have different native language backgrounds. During the few serious disagreements we've had, she would curse in my native language. Following your line of thinking, that would prove her excellent language mastery. Even if I'm on the receiving end of the cursing :-)