From https://www.mediawiki.org/wiki/Thread:Project:Support_desk/Localized_namespaces_not_working
Localized namespaces have stopped working in a MediaWiki installation: they are not translated into localized links, and localized links do not lead to the special pages. For example, for language "De", the entry "Kategorien" (categories) under special pages links to "Special:Categories" rather than to "Special:Kategorien". Also, opening the page "http://.../wiki/Special:Kategorien" gives "Spezialseite nicht vorhanden" (nosuchspecialpage: No such special page). The problem occurs both for the wiki's configured language in LocalSettings (Greek - El), and for a user-configured language (Deutsch - De) through user preferences.
I noticed this problem after upgrading from MediaWiki 1.19.3 to 1.24.0.
These are the software versions used: MediaWiki 1.24.0, PHP 5.3.5 (apache2handler), MySQL 5.1.55-log.