BCPL

BCPL is a general-purpose programming language with a small instruction set, primarily used for systems programming and compiler implementation.

Hello, world code example:

GET "libhdr"

LET start() = VALOF
{ writef("Hello world!")
  RESULTIS 0
}

Popularity: Less than 1% of developers are using or have used this language.*
*According to StackOverflow's 2023 survey.

• Wikipedia page