You can do an addon in strictly Lua. BUT, XML comes in handy when defining templates for the frames (IF I understand it correctly). It doesn't seem to be that difficult to
read XML, but writing it could be something else.
I don't, personally, write either one of those languages. I know what it says under my avatar, but the plug-ins that I "wrote" are, really, just copy-n-paste work. The graphics were the tough part (and they are simple graphics).