PL/I
PL/I (Programming Language One) is a high-level, general-purpose programming language that combines features of Fortran and COBOL. It is mainly used for scientific, engineering, and business applications.
Hello, world code example:
goodbye:proc options(main);
put list('Hello world!');
end goodbye;
Popularity: Less than 1% of developers are using or have used this language.*
*According to StackOverflow's 2023 survey.