use current i18n master in your rails app

# in case you have the gem already installed:
sudo gem uninstall git 

# clone the library and build the gem
cd /to/some/tmp/dir
git clone git://github.com/svenfuchs/i18n.git
cd i18n
rake build

# in case the build fails because of a broken gemspec file
# try to generate the gemspec and build again:
rake gemspec:generate 
rake build

# install the gem (you might need to adjust the version number):
sudo gem install pkg/i18n-0.2.0.gem

# create a rails app and freeze current 2-3-stable:
cd to/somewhere
git clone git://github.com/rails/rails.git vendor/rails
cd vendor/rails
git checkout --track -b 2-3-stable origin/2-3-stable

# As long as ticket #2871 (https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/2871)
# is still pending you have to apply the following patch manually:
curl -O https://rails.lighthouseapp.com/projects/8994/tickets/2871/a/219247/0001-relax-gem-version-requirement-for-I18n-gem.patch
git am 0001-relax-gem-version-requirement-for-I18n-gem.patch

# start the console and test if the gem is being used:
cd -
script/console
=> Loading development environment (Rails 2.3.3)
>> I18n # should always work
=> I18n
>> I18n::Backend::Base # should work with current master (after 0.2.0)
=> I18n::Backend::Base