Manual:$wgUseDatabaseMessages

Details
This variable determines whether the MediaWiki namespace is used to modify language files. If, then the wiki's interface is editable by those with   privileges; if false, then the wiki's interface is determined directly from the internationalized message file specified in.

Setting this variable to true will cause all interface messages to be retrieved from the database, which will increase load times by 25-60% (unless memcached is installed, in which case messages will be retrieved from the cache, which is a lot quicker). For performance reasons, it is recommended that you disable this setting if you do not have memcached.