MediaWiki is upgrading its plural rules to match CLDR version 26. The updates include incompatible changes for plural forms in Russian, Prussian, Tagalog, Manx and several languages that fall back to Russian [1]. In addition there are minor changes for other languages.

In January 2014, CLDR 24 had introduced several changes in the plural forms for some of these languages, including Russian, and we had updated MediaWiki's plural rules to comply with the CLDR standard. Some of these changes are now being reverted. Below is a detailed explanation of the changes.

For the migration period, from Monday, 27th October 2014 to Thursday 6th November 2014, we have disabled LocalisationUpdate at Wikimedia wikis to reduce the chance of ungrammatical translations being displayed in the interface.

Developers do not need to take special actions, but if you use master and do not update core, extensions and skins all at the same time, you might see ungrammatical translations in the languages mentioned above.

I recommend sysadmins to avoid mixing different versions of core and extensions for the same reason as above. If you have users in above languages and are also using LocalisationUpdate extension, you should consider disabling the extensions until your version of core includes the plural rule patch: https://gerrit.wikimedia.org/r/#/c/161920/

More details of the actual rule changes can be found at https://translatewiki.net/wiki/Thread:Support/Plural_rule_changes_for_many_l...

[1] Abkhaz (ab), Avaric (av), Bashkir (ba), Buryat (bxr), Chechen (ce), Crimean Tatar (crh-cyrl), Chuvash (cv), Inguish (inh), Komi-Permyak (koi), Karachay-Balkar (krc), Komi (kv), Lak (lbe), Lezghian (lez), Eastern Mari (mhr), Western Mari (mrj), Yakut (sah), Tatar (tt), Tatar-Cyrillic (tt-cyrl), Tuvinian (tyv), Udmurt (udm), Kalmyk (xal).

-Niklas