Localisation is in general the process of providing a Software with a multiple language user interface.
The Sidebar is an important navigation tool in the wiki world, as documented here.
The sidebar is now fully translatable with the Translation plugin usable for all the pages in the wiki.
This feature is a bit of an hack, so if you want some technical details on the way it has been implemented, you can refer to this post.
You can go to the special page : Special:Translate/wiki-sidebar to begin translation.
You will find most of the time 2 strings for each item in the menu. One represent the text who is displayed in the sidebar, and the other is the target of the link.
You can see the difference between the two by looking on the top of the 'translate box' where the name of the variable is displayed.