When you need to create a locale for a language variant (like Austrian for German), you probably don't want to duplicate your entire de.yml file only to change a few minor exceptions for our Austrian friends.

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Luckily, the I18n gem used by Rails has a fallback feature Show archive.org snapshot where you can make one locale file fall back to another if no translation is available.

In the example above you would have a config/locales/de_DE.yml:

de_DE:
  # hundreds of translations here

... and another locale config/locales/de_AT.yml:

de_AT:
  # only a handful exceptions here

Now write the following lines to config/initializers/i18n.rb to make de_AT fall back to de_DE:

require "i18n/backend/fallbacks"
I18n::Backend::Simple.send(:include, I18n::Backend::Fallbacks)
I18n.fallbacks.map('de_AT' => 'de_DE')

Check if a key is defined in the current locale (without fallback)

Do this:

I18n.exists?('my.key', fallback: false)

Rails 2

For Rails 2.3.11 you need to upgrade your project to use the I18n gem by adding this to the top of your Gemfile:

gem 'i18n'

Then run

bundle install