Ms. Gross’s white pad of notes, visible in photographs from the summit meeting, are probably useless, experienced government interpreters said, dictated in her personal shorthand that would be illegible to anyone else. And if she were to say what, exactly, transpired, she would violate an ethics code of confidentiality similar to lawyer-client privilege or the silence of a priest during confession.

Only Mr. Trump, who has alternately contradicted his own narrative of what was said and complained about a lack of fair coverage from a meeting only four people witnessed, could permit Ms. Gross to tell anyone about what she heard. The White House has not said whether Mr. Trump has asked her to do that.

“This is an absolutely nightmarish situation for anyone to be in,” said Stephanie van Reigersberg, who assigned interpreters to such meetings as the chief of the interpreting division in the State Department’s Office of Language Services for 18 years. “It’s a very difficult situation to be in, both in the point of her being asked to give information about what was a confidential meeting and because when you’re doing that kind of interpreting, there are memory issues.”