Basically, I want to to show the audience the result that the use of a plain language by someone who speaks English as a foreign language (non-phraseology communication) affects the aviation safety and a delay in the air traffic management.

For the resources and citation, you may seek a regulation from ICAO for language proffiency, ICAO convention or the specific company who flies to the US internationally requires their pilot to speak English at a certain level.

Note that there is a regulation for the standard requirements, but there is no standard test/ testing agency by ICAO to examine the specific requirement. I would like to point out that in conclusion, there is a problem as each testing agency has a different test materials, and it is hard to standardize and apply the same grading to all the pilots who speaks English as a foreign language. As a recommendation, I would like to say it is definitely a problem, and ICAO should have make a contract with an agency that makes the English test for every testing center for the fairness for all.