Of course, neither Kim Jong Un nor his ghostwriter said "dotard." The word shows up twice in the English version:
Action is the best option in treating the dotard who, hard of hearing, is uttering only what he wants to say.
I will surely and definitely tame the mentally deranged US dotard with fire.
The Korean, for those able to play along, was the following:
말귀를 알아듣지 못하고 제할소리만 하는 늙다리에게는 행동으로 보여주는것이 최선이다.
미국의 늙다리미치광이를 반드시,반드시 불로 다스릴것이다.
I don't really object to the translations. The statement clearly wanted to achieve hyperbole, and there's no way to soften that and give a true translation. The word 늙다리, translated as "dotard" doesn't really have a better translation. Other ones might have been equally good, though: old coot, buzzard, dinosaur, etc. It's an old person whose age has weakened his/her faculties. (Did that many people really have to look up the meaning of "dotard" in English? That was surprising to me.)
It's not that unusual a word in Korean, though. It can be used about anything old. An old animal that's past its prime can be a 늙다리. I think most adult Koreans would know what the word means, unlike (much to my surprise) most Americans with the word "dotard." It isn't, in other words, a particularly strange thing to say in Korean. On the scale of KCNA pronouncements, it's actually kind of normal.
I wouldn't begrudge folks having a laugh over such a pedantic point, except that it seems like the U.S. media has only one note with North Korea, which is to make fun of everything it does or says. It's all just a big joke, all the time. Well, it's not a joke to North Korea. It's an existential matter for them. Nobody there is laughing.