TeX wins "World's Most Narrowly Useful Programming Language" hands-down. Macro languages enjoyed some popularity in the 1970s (think macro assemblers, M4, TRAC, and others). They've dropped out of most common use. The most common today of my examples is probably M4, which gets used in Sendmail configuration files and in GNU's autoconf system. Not exactly widespread use.
The C programming language's "cpp" preprocessor isn't really Turing complete, so while used it cannot count as a programming language.
Except that one macro language enjoys a large user base, new applications, and wide use: Donald E. Knuth's TeX. Knuth recognised that the chief failing of macro languages -- that anything not classified as a command gets silently ignored (so there aren't enough diagnostics for common programming) -- is an advantage when setting text. After all, in a paper most of the text is plain and not marked up. By "stealing" an uncommon character (the backslash \) as a command prefix, the macro processor is able to report many errors. For programming, all those backslashes are annoying, but for marking text they're just what the doctor ordered.
TeX is, of course, Turing complete. There's even a toy implementation of BASIC in TeX. The most common use of TeX is through LaTeX, an entire typesetting application written in TeX! There are many packages which extend LaTeX, too.
Despite all this, TeX never gets used for anything except typesetting. It's a complete programming language, big applications have been implemented with it, and it's only ever used in the one role. One of the world's most useful programming languages, and certainly one of the world's most narrowly useful programming languages!