• providing support for English and similar languages out of the box.
• making it easy to customize and extend everything for other languages.

To achive this Ruby i18n gem spilt in two parts:

(a) public API of the i18n framework: Ruby module with public methods that define how the library works.
(b) A default backend: It implements these methods.

Now lets switch to the real task of how to configure i18n module in rails app. Rails sets up application with reasonable defaults, that you can override. Rails adds all .rb and .yml files from config/locales directory to your translations load path automatically. The default file available there, is en.yml for english. If you look into this you will find:

en:
  hello: "Hello World"

Here key hello maps to the string "Hello World". If you want other languages, you can add them too here. Thus those languages will also be added to translations load path. You can get a list of those files here at https://github.com/svenfuchs/rails-i18n/tree/master/rails locale. You can also use YAML or standard Ruby Hashes to store translations in the default backend.
The translation load path is just a ruby array of paths to be automatically loaded and added to the application. To load locals from another directly, application.rb file provides instructions. For that uncomment the given files in application.rb and edit them.

# The default locale is :en and all translations from config/locales/*.rb,yml are auto loaded.
# config.i18n.load_path += Dir[Rails.root.join('my', 'locales', '*.{rb,yml}').to_s]
# config.i18n.default_locale = :de

There is another way of configuring i18n, without even touching application.rb. You need to set the load path anywhere in the application, where it can run before the translations are looked up. Best thing is to put them into initializers.

# in config/initializers/locale.rb
 
# tell the I18n library where to find your translations
I18n.load_path += Dir[Rails.root.join('lib', 'locale', '*.{rb,yml}')]
 
# set default locale to something other than :en
I18n.default_locale = :pt

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