True, poor choice of words ;) more than slang it is colloquial Spanish. It is not only used in the south or by less educated Spaniards though, everybody from peasants to the king would eventually use "ná", "comprao" or words like that. Think of Spanish mothers when their children don't want to it something, a slightly blackened banana for instance: "¡venga, que a eso no le pasa ná!" :D
There you have the Spanish king saying "me han tratao" instead of "me han tratado" ;)
You are right with "ná de ná", that specific expression is more like a joke, but "ná" instead of "nada" is very common, and lots of Spaniards use it in daily life. One would avoid it if the situation doesn't really allow its use, (say a work interview, for instance), but you can hear it pretty much everywhere in pretty much every context. Bad Spanish, alright, but very normal Spanish as well.
"Trata--(d)--o" is not "tratao". Many people use a very soft "d" but the "d" IS there. And the king is not a good example of good use of language... nor of anything else: he was apologizing to Spain in that video.... "Lo siento, no volverá a pasar...." People who say clearly "tratao" are from the south, or they are people whose parents are from the south. It sounds awful in the North, low class and vulgar. It sounds completely normal in the south. If you are in a bar of the Andalussian Association in any north of Spain city, you could hear tratao, yes. But it's not colloquial Spanish. Cheers!