Just to give my experience, I actually didn't know what that acronym meant. I tried googling it but "ESL speaker" only gave results about loudspeakers and just "ESL" was mostly about some German Company.

I even thought it had to do with sign language at first but didn't get the meaning of the sentence in that case!

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A little off-topic but that makes me think of another HN thread some months ago talking about acronyms like "SRE" or "SWE" (and some others I since forgot), that many HNers assumed to be understood by everyone. Many non-american/english developers actually never encountered those (including I, I've been software engineer since at least 8 years and never saw those terms beside HN, like I would think most in my country) and it led to the same kind of incomprehension.