WebAssembly does have GC, Polymorphic Inline Cache, and direct DOM integration on their roadmap.
So, at some point, you should be able to compile any language to WASM, including Javascript. The only real barrier would be the size of the runtime / standard library for the language. If WASM supports also supports some kind of hashed cache where many sites can share a language runtime, that's not even much of a barrier.
I suspect once it's stable, you'll see all sorts of languages being used in the browser.
So, at some point, you should be able to compile any language to WASM, including Javascript. The only real barrier would be the size of the runtime / standard library for the language. If WASM supports also supports some kind of hashed cache where many sites can share a language runtime, that's not even much of a barrier.
I suspect once it's stable, you'll see all sorts of languages being used in the browser.