In the past couple of months, I've had an opportunity to witness two of my colleagues interacting with students in a small-group discussion kind of setting and what I saw kind of disturbed me. Even though the students were somewhat high-level (maybe low-intermediate +), the teachers made no attempt whatsoever to grade their language. You can grade your language in 2 ways:
1. Speed. Just use whatever grammar/vocab you want, but speak more slowly. And use pauses to allow for some thinking time.
2. Difficulty, in terms of grammar/vocab. Make it simpler for lower-level students and more difficult for higher-level ones.
These two teachers didn't grade in terms of speed or difficulty and just talked to their students like they would have talked to me, a native English speaker. The result is that the students had the "deer in headlights" look in their eyes and really didn't know how to interject any comment into the "conversation." The result was the teacher perhaps felt awkward and then just kept talking, but ended up just making worse because instead of the 1 minute monologue the students didn't understand, it had now been upgraded to a 3 minute monologue.
Now, if the teacher had noticed the confused looks in her/his student's eyes, she/he could have thought, "Wow! My students have no idea what I'm talking about right now" and then simplified/ summarized what she/he had just been saying. Except, of course this wasn't the case. The "teacher" who would do this in the first place probably is not the one who is really caring whether or not the students are "getting it."
So many Koreans have commented that they can easily understand my "accent." It's not the accent, it's just that I grade my language, but they don't know how to say this. Think about it!