You expect Putin of the KGB to presume the person he's speaking to doesn't have a recording device or the ability to remember the words he used and have them translated by someone else later? And if he did think that, wouldn't that be an advantage to the US of doing it that way, to capitalize on the chance he lets his guard down? And if he did let his guard down and Trump failed to capitalize on it, what disadvantage would the US suffer from that compared with the situation where he knows there is a US translator in the room and doesn't say it to begin with?
You expect the Russians to strip search the President of the United States to check for a listening device when they know that person will be in the room to hear whatever is said anyway, and the US to agree to that?
At which point they can tell that he wears a watch but not whether the watch is a recording device, and can't detect shielded devices that can record audio onto internal storage with no RF emissions, and still can't do anything if he remembers a phrase and asks for it to be translated later.