It's been weeks now since I looked through a language, called S-SIZE, filetype .ssz) that is so little used around (I think it's only used is for a game engine clone called Ikemen) that I can't find much documentation or talk about it on Google.
I'm kinda surprised actually. Is it kind of a common thing to see languages used for a single application?

That's a domain specific language, and yes, there are tons of languages out there that were developed for very specific uses. A lot of them become obsolete over time as usage declines.

Fun fact, a lot of the larger companies run DSLs for their internal systems, though now the trend seems to be shifting towards embeddeding that DSL functionality inside a general language like Haskell or Python.

@RememberMe Got it. Thank you very much for the explanation :)