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.
There is a bug with the Download page. You can't redirect the link to "Download/lang". Instead, the link will point to the actual translation of "Download" in your language. The best way to deal with it is to create this new page and do a redirection to the right page. More about redirection on Help:Editing.
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. When the variable name ends with "-target", it mean that you are translating a target link. It's made to allow the allow the translator to redirect the items to the translated pages (by adding "/fr" after the name page for french translation for exemple).
DO NOT add the "/lang" if the translate page doesn't exists in you langage, it will broke the link. DO NOT write anything else than the name of the page or it will broke the link.