It seems that all of a sudden all the Wikipedias shown their UI in English when users are not logged in ?
Tested with incognito on:
https://ru.wikipedia.org/wiki/Википедия:Hauptseite
https://de.wikipedia.org/wiki/Wikipedia:Hauptseite
There are also multiple complaints here:
https://en.wikipedia.org/wiki/Wikipedia:Help_desk#Sidebar_changes