It's not trivial because you have to figure out intricacies of the language and whatever compiled it into WASM, and Emscripten compiled WASM does expect some JS glue code. But WASM with WASI doesn't inherently require JS. And since Emscripten's JS glue is called via WASM host imports, you can implement them in whatever the host language is.
https://news.ycombinator.com/item?id=38146154
It's not trivial because you have to figure out intricacies of the language and whatever compiled it into WASM, and Emscripten compiled WASM does expect some JS glue code. But WASM with WASI doesn't inherently require JS. And since Emscripten's JS glue is called via WASM host imports, you can implement them in whatever the host language is.