Everyday I get more surprised about the native language level of people. It looks like the habit of everyday talking tends to negatively bias their language level.
I’ve seen around some similar questions, but they are not exactly what I’m asking. We may also find a clue if we check a nation’s education level, but a high level exam is more restrictive than a basic education test.
Let’s suppose we are talking about USA, how would USA inhabitants do when they face a Proficiency exam or any of similar level?