Which programming language has the highest meaning density?

I assume you mean information density? If so programming languages basic information density is set by their amount of keywords and symbols used for parsing. But there also is implicit density caused by every abstraction - therefor a Lisp can express extremely complex programs with only a few lines of code. This may also hold true for natural languages when using phrases or metaphors.

https://www.science.org/content/article/human-speech-may-hav...

I wonder if something similar applies to code. Like APL etc are much more compact, but are they written and read more quickly?

>Meaning that if a child studies in Mandarin for 15 years of her life, she could have learned as much as 75 years worth of knowledge

That's the sort of claim that requires extraordinary evidence.

Yeah. It assumes that reading bandwidth is the limiting factor in how fast children learn. Not brain development, not actually getting the fundamental concepts; just reading.

I don't buy it at all.

What does “meaning density” mean?

K? APL?