Please not, Lua would be easy to implement but if we can only pick one language then it's too similar to JS. We need more diversity, static types+AOT+noGC would be a good start.

Also if we do get Swift it will be for the same reason we got <canvas>. Apple wanted to make dashboard, so they put an extension in webkit, people started using it on the open web and...

Lua would certainly be a good choice. A really good language syntax, the language is small, and LuaJIT2 is very very fast.

Though, it's better to stay with one language, JavaScript - it's a good language, and everyone should learn it first (I would argue most simply to don't understand Javascript and use only a fraction of it, in an non-optimal way). Fragmentation is bad for the web. I would already consider CoffeeScript/TypeScript/Dart/etc that compile to JS bad because its fragmenting the developer base and it makes it harder to reuse code. Have you ever visited a GitHub page only to find out that a lovely open source project is coded in some obscure language that compiles to Javascript.

I've been waiting for browsers to implement this for years, would love the opportunity to drop JS for Lua!