Obviously if I were in a relationship with someone who didn't speak English natively, I'd learn their language; that's a given. But more because of the "language! shiny!" factor than the relationship part.

My uncle, however, married a woman from Vietnam and to the best of my knowledge hasn't made any attempt to learn Vietnamese. Their kids are being raised bilingual, though. (Since the primary babysitters are her parents, whose English is minimal, it doesn't matter if their father doesn't ever use it--they still get plenty of practice.)