Hello, again me.
In our projects (redmines 3.2.0) we are using russian language sometimes. And sometimes we have problems with int plural in words (In our own others projects everything fine).
I know how to fix this, just need to implemet CLDR rules. I figured out how to do it, but yours backend of i18n breaks all my dreams ^.
So, please, restore i18n logic. There is lot of related issues to #12645 in redmine.org, but you still does not fix this simple miss.

Why this happened:
I18n::Backend::Base which you are include to module Implementation of Redmine::I18n::Backend contains pluralize method only for English language (:one or :other from locales).

How to:
In module Implementation of Redmine::I18n::Backend class need to add this:

include ::I18n::Backend::Pluralization

Original implementation of I18n::Backend

    autoload :Base,                  'i18n/backend/base'
    autoload :InterpolationCompiler, 'i18n/backend/interpolation_compiler'
    autoload :Cache,                 'i18n/backend/cache'
    autoload :Cascade,               'i18n/backend/cascade'
    autoload :Chain,                 'i18n/backend/chain'
    autoload :Fallbacks,             'i18n/backend/fallbacks'
    autoload :Flatten,               'i18n/backend/flatten'
    autoload :Gettext,               'i18n/backend/gettext'
    autoload :KeyValue,              'i18n/backend/key_value'
    autoload :Memoize,               'i18n/backend/memoize'
    autoload :Metadata,              'i18n/backend/metadata'
    autoload :Pluralization,         'i18n/backend/pluralization'
    autoload :Simple,                'i18n/backend/simple'
    autoload :Transliterator,        'i18n/backend/transliterator'

As you can see, there is Pluralization exists.
I18n::Backend::Pluralization contains rails logic for extandable plural for different languages. We can easily place our ru.rb file to config\locales and write some simple logic from CLDR.

Thats all.

Today i just wrote small patch for one of our plugin, but it s stupid, we need general solution.

Thanks in advance.