Per discussion in #15920, I’ve figured out that in Russian we need 4 plural forms instead of 3 to distinguish a single item from 21, 31, etc. items in certain cases.
I’ve changed the Plural-Forms header in .po files to include the rule for a single item:
"Plural-Forms: nplurals=4; plural=(n==1) ? 0 : (n%10==1 && n%100!=11) ? 1 : ((n%10>=2 && n%10<=4 && (n%100=20)) ? 2 : 3);n"
and it works fine with Poedit. However when I import the translations into GlotPress, there are still only 3 forms. I guess the header is stored elsewhere.
Is it possible to update it in GlotPress both on translate.wordpress.org and translate.wordpress.com?