I think we're worried about the Trump administration lying about what they talked to the Russians about. A translator with loyalty to the USA might request what they heard.
Which is an even more fanciful conspiracy theory since the objection was Trump not bringing his own translator, a person he could otherwise personally select for their loyalty to him.
> 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.
Translators are civil servants with security clearances, not reporters. When the leaders of Germany and Brazil have a meeting, they may say things they don't intend for China and everyone else to know about, and it isn't the translator's job to report what was said to the media. Unless they've witnessed something on the scale of criminality that they need to act as a whistleblower (in which case they wouldn't need anyone's permission), they should never be telling anyone what happened in the room since their job is to keep those secrets.
The media are fully aware of that, but they're being disingenuous and pretending that allowing a diplomatic translator to act as a media witness is a request anyone is likely to grant. Then Trump predictably responds to that by removing the translator from the room, so now they're complaining about that instead.