Manual:$wgCapitalLinks

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.

On Wiktionary projects of all languages, this setting is disabled, but enabled on all other Wikimedia projects.