Manual:$wgUseDatabaseMessages

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

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.