A lot of Americans have a very poor understanding of their own language. I"ve given up listening to contempary music, full of 'aint's', female dogs, f's every 5 seconds and so on. Must be the no-child-left-behind policies.
As I said earlier, Filipino teachers are in higher demand in Thailand than American teachers, and for a good reason. Americans don't travel much, but if they do and teach abroad, they expect their students to know the American football rules, celebrate the 4th of July,and even enjoy Halloween.
https://www.migrationpolicy.org/article/limited-english-proficient-population-united-states-2013
https://www.popularmechanics.com/science/a43469569/american-iq-scores-decline-reverse-flynn-effect/