It’s coming from the pluralization rules:
https://github.com/discourse/discourse/blob/master/lib/javascripts/locale/ru.js
The Russian locale uses one, few and other.
The English locale uses one and other.
Transifex doesn’t detect a string as pluralized when the English locale file contains pluralizations different from one and other (e.g. it uses zero). That’s why few is not available in Transifex for those strings in the Russian locale.
I don’t think there’s a need for that. They already support up to 6 plural forms and all the supported languages should be configured correctly on their side.
If I’m not mistaken removing all the zero and many keys from the English locale should be enough.