As far as I remember Lua scripting is brought by the lua-ngx module, but it is also possible to use JS or the JVM (ngx-clojure) for location handler, or even write your own nginx module in C, nevertheless Lua is the first class citizen. I personally don't mind it, but obviously the ecosystem and the market for Lua is much smaller than JS. In fact, I'm only aware of itch.io that runs on OpenResty.