This is an example module but more than code it is a guide it has many steps that you can follow to make your custom module
Compatible with v2 pre7 onwards
The code will soon be 100% free on github and it will not be necessary to buy it, if you want you can do it but it is not necessary.
This module / guide will help you to create
Understand a little about cache and smarty variables,
Create links in the navbar (Navbar and sliderbar on panel),
Create a special section in the profile
and finally how to create a custom language