Hallo,

Today the Language Engineering team merged a change in MediaWiki, which brings MediaWiki up to date with the latest version of the CLDR standard for plural rules support. The general idea is to make plural support more robust and standard and less dependent on our own code.

The relevant Gerrit changes are here and they have a detailed description of the changes:

Russian: https://gerrit.wikimedia.org/r/#/c/105148/
Other languages: https://gerrit.wikimedia.org/r/#/c/104795/

Most notably, these changes affect Russian and some other Slavic languages: Ukrainian, Belarusian, Serbian, and also some languages that have Russian as fallback and don't have their own rules. See the details here:
https://translatewiki.net/wiki/Thread:Portal_talk:Ru/Plural_changes_in_many_languages

The change will be deployed next week, so please take a look at the changes and the translations ASAP.