Seems to not be Wikimedia-only, I experience this error too:
| Product | version
| MediaWiki | 1.25.2 (9c89a16) 18:08, 3 September 2015
| HHVM | 3.8.1 (srv)
| MariaDB | 10.0.20-MariaDB-0+deb8u1-log
Configuration: https://github.com/miraheze/mw-config/blob/master/LocalSettings.php
```
#0 /srv/mediawiki/w/includes/StubObject.php(204): RequestContext->getLanguage()
#1 /srv/mediawiki/w/includes/StubObject.php(160): StubUserLang->_newObject()
#2 /srv/mediawiki/w/includes/parser/ParserOptions.php(588): StubObject->_unstub()
#3 /srv/mediawiki/w/includes/cache/MessageCache.php(148): ParserOptions->__construct()
#4 /srv/mediawiki/w/includes/cache/MessageCache.php(988): MessageCache->getParserOptions()
#5 /srv/mediawiki/w/includes/Message.php(1059): MessageCache->transform()
#6 /srv/mediawiki/w/includes/Message.php(726): Message->transformText()
#7 /srv/mediawiki/w/includes/Message.php(789): Message->toString()
#8 /srv/mediawiki/w/includes/User.php(748): Message->text()
#9 /srv/mediawiki/w/includes/User.php(790): User::isUsableName()
#10 /srv/mediawiki/w/extensions/CentralAuth/includes/CentralAuthHooks.php(1082): User::isCreatableName()
#11 /srv/mediawiki/w/extensions/CentralAuth/includes/CentralAuthHooks.php(835): CentralAuthHooks::attemptAddUser()
#12 /srv/mediawiki/w/includes/Hooks.php(209): CentralAuthHooks::onUserLoadFromSession()
#13 /srv/mediawiki/w/includes/User.php(1132): Hooks::run()
#14 /srv/mediawiki/w/includes/User.php(361): User->loadFromSession()
#15 /srv/mediawiki/w/includes/User.php(4942): User->load()
#16 /srv/mediawiki/w/includes/User.php(2587): User->loadOptions()
#17 /srv/mediawiki/w/includes/context/RequestContext.php(342): User->getOption()
#18 /srv/mediawiki/w/includes/Message.php(577): RequestContext->getLanguage()
#19 /srv/mediawiki/w/includes/context/RequestContext.php(436): Message->setContext()
#20 /srv/mediawiki/w/includes/context/ContextSource.php(176): RequestContext->msg()
#21 /srv/mediawiki/w/includes/OutputPage.php(974): ContextSource->msg()
#22 /srv/mediawiki/w/includes/page/Article.php(512): OutputPage->setPageTitle()
#23 /srv/mediawiki/w/includes/actions/ViewAction.php(44): Article->view()
#24 /srv/mediawiki/w/includes/MediaWiki.php(395): ViewAction->show()
#25 /srv/mediawiki/w/includes/MediaWiki.php(273): MediaWiki->performAction()
#26 /srv/mediawiki/w/includes/MediaWiki.php(566): MediaWiki->performRequest()
#27 /srv/mediawiki/w/includes/MediaWiki.php(414): MediaWiki->main()
#28 /srv/mediawiki/w/index.php(41): MediaWiki->run()
#29 {main}
```