Interesting observations, H.J. I have found the same thing, having spent about ten years with students originating from the same region, each undergoing similar full-time courses.
Likewise, our clients underwent the "aviation English" course, which failed to account for a satisfactory level of comprehension. However, the economic situation has since seen the abolition of the course, and removal of any specified level of English competency prior to the commencement of training. Doubtless the token gesture was substantially better than its complete absence.
Given that the only formal checkpoint for English competency is currently the issue of the SPL, the pressure is now placed wholly on the CFI. This is frequently more an economic consideration, than anything else, despite the better moral judgement, otherwise exercised.
Unfortunately, I have found the "say again" option is rarely exercised here, at Bankstown, and its surrounds. Mostly there is just the "clipped callsign response", despite the feverish attempts of ATC, FIS or other aircrew to elicit a sensible answer to a simple question. Seemingly it is difficult to change the cultural aspects accompanying language barriers, which might otherwise allow the individual to accept a potential or actual mistake.
Regardless, I, too, take my hat off to the guys and gals from o/s, who battle with English and achieve the standard in the end. Despite my empathy and the endless hours of training, an acceptable and measureable English standard is very much in order, methinks.
Tks for your willingness to help with the research, too. I'll be in touch. Cheers!