It begain with his first wife Ivana referring to him that way back in the 70s or early 80s when they first started getting a lot of society press interviews. It made "cute" headlines in NYCity. I am speculating that it is a figure of speech in her native Czech language, because the same thing is done in Italian. It is a polite way to confer respect on someone, yet not go all the way to just the first name. Sort of halfway between "Mr. Trump" and "Donald."
The closest we have in American English is the way the Southerners use "Mr" or "Miss" with a person's first name to show both familiarlity and respect at the same time; i.e. "Driving Miss Daisy."
OTOH....I DO know what it is in Hungarian ( which is NOT to call someone "THE" before a name ) and since the n ow Czech Republic was once parft of Hungary, I doubt that this is the case.
English is Ivana's fourth language, she was struggling to learn English, when she and The Donald me and later wed, so my guess is that she was just struggling with English, still at the time of all of the interviews. And yes, it was considered to be "cute" and even "endearing" back then.
I find it amazing how many FReepers that “know” so much, in reality, know so little.