Why does Trump use such basic language when a man in his position should be capable of speaking at a higher level?

I saw an interview with Trump from a few years back. He had good language skills and spoke at an advanced level. Now he speaks at about a middle school level (IMO). I just saw him on the news talking about North Korea and he said “They better get their act together or they’re going to be in trouble like few nations have ever been in trouble”. Ok, I’m thinking 4th grade speaking level now.

Does he have dementia or Alzheimer’s disease? If not, what could cause his speaking level to go so far down?