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/

Now that piques my interest. I really enjoy writing Vala.