"If you talk to a man in a language he understands, that goes to his head. If you talk to him in his language, that goes to his heart."
I really love this quote by Nelson Mandela, and I think it goes in two different directions. First, if you want to communicate most effectively with others, learn their language of the heart. I'm not saying you can do to this all the time (I fizzled out with Spanish after doing Arabic and Russian), but it makes a difference. Learn the language if you can but be realistic.
Second, second languages can never replace native languages. I think this is especially important in teaching the gospel and even though we can hope people learn new languages and are able communicate in non-native languages, people need to be able to pray, read the scriptures, and teach in the language of their heart.
Sometimes people's language of the heart changes. But not very often. We need to respect that language more.