All WoW addons use lua as the language.
The only required XML is in secure templates. Everything else can be accomplished with XML and lua or just lua (XML is only used for defining frame structures and everything there has a lua equivalent).
The code above appears to be from Tchïn Raid Scheduler (at least according to the title text)
I have no idea if that addon still exists but if you were going to use it as an example, at least place local at the start of the first line. Anything defined without a local declaration is global in scope and all addons and all of the Blizzard UI share the same global space.
local UIConfig = CreateFrame("Frame", "Tchin_Frame", UIParent, "BasicFrameTemplateWithInset");
All addons are text so there’s nothing stopping you looking and learning. Copying/pasting great chunks of code is pretty much frowned upon but using other code as the basis for your own addon idea is fine. Also people will help with code that you can use if you outline the idea of what you are trying to create (or better yet, have a go and post the code that’s giving problems).
Anyting inside a print() goes to you chatbox only.
There’s a reference to the WoW API here:
With more about EditBoxes Here:
The code for an editbox being used along with a scrollframe is here (feel free to copy chunks of this one 
Asking questions is generally the best way to start.