The Vector Skin for MediaWiki 1.35 does not show the CategoryTree portlet and the LanguageSelector portlets in the MediaWiki:Sidebar
Problem was initially reported as a collapse and expand icon missing issue in the Support Desk
https://www.mediawiki.org/wiki/Topic:Vylqwkx9geua4l7v
which I had resolved by installing the Vector Skin from MW 1.34 The icons now show when I upgraded to MW 1.35.1 however the CategoryTree and LanguageSelector do not show in the MediaWiki sidebar which is coded as
* Navigation
** mainpage|mainpage-description
** Special:RequestAccount|RequestAccount
** recentchanges-url|recentchanges
** helppage|help
** TNG_Wiki map|TNG Wiki Sitemap
* languageselector
* SEARCH
*TNG Guides
** Category:Getting Started|Getting Started
** Category:Visitors|Visitors
** Category:Registered_Users|Registered Users
** Category:Administrator|Administrator
** Category:Programmer|Programmer
** Category:Mod Manager|Mod Manager
** TNG_Glossary|TNG Glossary
*categorytree-portlet
...
So I had to revert back to using the Vector Skin from MW 1.34
We use the CollapsibleVector extension to allow the CategoryTree and other sections of the sidebar to be collapsed