C++, the language so readable that every character has to be scanned averagely 7 times by the compiler. Even computers have to bend backwards to parse it, now throw humans into the loop an hilarity ensues.
@raichoo Also the C++ grammar (i.e. compiling C++) is undecidable.
There's actually surprisingly many languages for which this is the case, see (shameless plug) https://typing-is-hard.ch
Everytime you look into it it generates a new wheel, it has long since go beyond just square.