Mostly true, but there are languages which are completely useless without becoming proficient with them. In my experience Forth and J are such languages: you can "learn about" them, but you won't really understand them without using them for some time. So I suspect APL, K, Q and Eli here are similar; the question is what will I gain from becoming proficient with one of them or alternatively is it worth becoming proficient in one of them if I'm already proficient with another.