I cannot speak to the specifics of how a U.S. officer deals with that. I could explain how a CBSA officer would deal with that.
In interaction with a traveller, you want to make sure that there is an understanding. In those cases where we have somebody who presents himself or herself in front of us, who cannot speak either of our official languages, then we have translators available and have the ability to reach out to those people to make sure there is a clear understanding.