MediaWiki generally assumes everything is NFC. However in certain cases, it appears that $wgLang->ucFirst() might cause normalized unicode to become unnormalized, which can cause some weird affects.
First: e.g. https://en.wikipedia.org/wiki/Greek_Extended - characters like ῷ or ῧ despite the fact that if you click on them you get to a real article. Its because Title is uppercasing them incorrectly, but when you go to the page, MW normalizes url parameters.
Second: if you go to a page like https://en.wikipedia.org/w/index.php?title=%E1%BF%B7&foo it shows you a missing page, however if you click create, then the page exists. This is because MediaWiki uppercases %E1%BF%B7 (U+1ff7 Greek Small Letter Omega with Perispomeni and Ypogegrammeni.) to %CE%A9%CD%82%CD%85 ( Greek Capital Letter Omega + Combining Greek Perispomeni + Combining Greek Ypogegrammeni.) but the correct uppercase would be %E1%BF%BC%CD%82" (Greek Capital Letter Omega with Prosgegrammeni + Combining Greek Perispomeni )
