Metafont
Metafont is a programming language developed by Donald Knuth used to generate high-quality fonts with precise control over their shapes and sizes. It is primarily used in the creation of mathematical, scientific, and customized fonts.
Hello, world code example:
message "Hello world!"; end
Popularity: Less than 1% of developers are using or have used this language.*
*According to StackOverflow's 2023 survey.