But readability has a lot to do with what you are used to.
The only exception might be FORTH. A very well written FORTH implementation (and I mean very well written) probably would be fairly readable to anyone — at least at the higher levels of abstraction.
"In 1968, while employed at the United States National Radio Astronomy Observatory (NRAO), Moore invented the initial version of the Forth language to help control radio telescopes."
I know of one: the U.S. Naval Observatory near Flagstaff, Arizona. I lived in Flagstaff in 1979. I knew the astronomer and visited him at the observatory at night. He showed me a terminal running Forth that could control the telescope.
That FORTH cannot muster the decency to include a simple «U» in its name rather scuppers any hope of an aesthetic parley before the tea’s even been poured.
Joking aside, FORTH’s reliance on the stack as virtually its only data structure – along with its insistence on applying it to everything and everyone – is simply too impractical outside a few niche areas, such as low-level hardware programming or document rendering (hello, PostScript!). I have no doubt a JSON parser in FORTH will emerge as part of Advent of Code 2038, but I can’t imagine it will inspire hesitating potential converts to embrace the language with open arms.
I’ve not used FORTH, but I learned UIUA last year which has the stack paradigm. You just have to be more careful about your function design and the order of application. I was using emacs and there were annotations by the language server that said how many items you’re taking from the stack and how many will stay behind.
But readability has a lot to do with what you are used to.
The only exception might be FORTH. A very well written FORTH implementation (and I mean very well written) probably would be fairly readable to anyone — at least at the higher levels of abstraction.