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 retrieved from the corresponding language??.php each time it looks for a message; otherwise, the cached copy will be returned.

It is not recommended that this is used without memcached.