Can't tell a darn thing from the linked page or the home page. What language? What platforms? Either I'm really dumb or the page is incomprehensibly designed with no actionable information.

The "What is it?" section of the introduction in the manual is informative and answers your "platforms?" question. Reading between the lines it seems like Lua is the only supported language.

https://docs.crownengine.org/html/latest/introduction.html

The Github page suggests the engine itself is mostly written in C++ and Vala.

https://github.com/crownengine/crown/