Manual:$wgUseDatabaseMessages

Language, regional and character encoding settings: $wgUseDatabaseMessages
Translation using MediaWiki: namespace.
Introduced in version:	1.1.0
Removed in version:	Still in use
Allowed values:	(boolean)
Default value:	true (1.2+) false (1.1)
Other settings: Alphabetical | By function

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.