This is the username used by the maintenance script rebuildmessages.php. The script is part of MediaWiki; it updates MediaWiki interface messages to be equal to the MediaWiki default. It is not a bot and is unaffected by blocks.
The script has two modes of operation:
rebuildMessages.php --update- Updates any messages which were last edited by "MediaWiki default" to their current version. Creates any messages which did not exist.
rebuildMessages.php --rebuild- Updates all messages to their current default value, regardless of whether they have been manually edited. This is sometimes used on other wikis when a new messages file has just been created, with a comprehensive set of default messages. It has never been used on the English Wikipedia.
Edits made by this script have a user ID of zero, so they are treated as anonymous edits in the page history, with a link to the user contributions instead of the user page. Therefore they cannot be viewed through the contributions page due to T36873. They can still be viewed through the API, like this.
The same username is also used by the MediaWiki installer. Installing MediaWiki has the same effect as rebuildMessages.php --update.
If there is a problem with the text of the default messages, please file a bug report at https://phabricator.wikimedia.org. Requests to start or stop the script should be made to the #wikimedia-tech channel on irc.freenode.net.