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.

This will increase load times by 25-60% unless memcached is installed. Interface messages will be got from the database.

If set to false, message information will be get from corresponding language??.php each time it looks for message, otherwise, the cached copy will be returned.

It is not recommended that this is used without memcached.