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

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.