We as customers are not given the option to write smartapps that will run on the hub. And there will no longer be a publicly available Samsung smartthings cloud where they can run either.
So you can write a smartapp in any language you want, JavaScript, python, LUA, even groovy: but you are going to have to host it yourself. Either through a hosting service or on a local server device. That smartapp will then call the rest API to send requests or get information from smartthings.
(Lua is really only significant if you want to write a custom edge driver, which would be the replacement for a custom DTH. Those need to be written in Lua. )
Here’s the original getting started tutorial: I don’t know if there’s something newer or better. Hopefully someone else will be able to comment on that.
And here’s an example that sounds like it might be somewhat similar to your use case. But again, I don’t know if there’s any newer or better documentation now, hopefully someone else will know.