Manual:$wgCapitalLinks/en

Details
Set this to false to avoid forcing the first letter of page titles (including included pages, images and categories) to capitals. WARNING: may break links! This makes links COMPLETELY case-sensitive. Links appearing with a capital at the beginning of a sentence will not go to the same place as links in the middle of a sentence using a lowercase initial; typically the former has to become a link with a separate display title (e.g. page ). After setting this to false, run to fix the existing links that will be broken.

This settings affects all  except Special, (and as of 1.16) MediaWiki and User. In versions before 1.16, it affects the MediaWiki namespace, but remember that MediaWiki only recognizes pages in upper case. So if you for example edit "MediaWiki:sidebar" you will see nothing has changed, but editing "MediaWiki:Sidebar" works normally.

can override this setting on a per namespace basis.