No scripting language, most of the heavy lifting done in plugin is through shell calls (asynchronously or not). Example here [1]. I personally think that it's kinda hard to grok to be honest.
Thanks to the client server architecture, it is possible to make an external UI[2].
There is no line wrapping currently. Don't know about long documents, but the editor in general is very efficient.
They are just command that you could execute manually (using ':' like in vim). The difference is that all the commands in this file are executed at startup.
As I said, all the heavy lifting is done in scopes like
%sh{ ... }. This way you can actually use any language you want, and it's only the piping you have to do through kakoune commands.
Most common languages (like go) already have plugins to do some basic tasks like formatting.
If you're interested in the editor, I would rather recommend reading the readme of the project[1]. There is a lot to read, and you can start making plugins once you understand it.
This is an important note to me, because i imagine i'll have to implement my own tools for common language tasks like Gofmt and Goimport.
Honestly, it would be cool if i could write plugins in any language i want, ie if it fully uses a backend editor server API.