In all seriousness, this is important. I am convinced that a person cannot truly understand a culture without mastering the dominant language. A our intelligence performance in the Mideast has shown, you cannot just rely on native speakers who may not fully understand American culture or who may not accept or support American culture. At different periods of my life, I have learned Spanish and Thai and my experience has been that the manner of communication, word patterns, politeness, cures, humor, etc. all flesh our and explain much about beliefs, social structure, power, and the like. "Have them learn English" may allow them to communicte with us but not allow us to communicate with them.
Don't you hope...
Let's just hope that the Chinese nanny is more pro-American than Dad is...