consoleからI18n.load_pathとすればよい。外部のgem経由で読み込まれているファイルも含めて、arrayで返ってくる。
[1] pry(main)> I18n.load_path
=> ["/Users/foo/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activesupport-3.2.8/lib/active_support/locale/en.yml",
"/Users/foo/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activemodel-3.2.8/lib/active_model/locale/en.yml",
"/Users/foo/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/activerecord-3.2.8/lib/active_record/locale/en.yml",
"/Users/foo/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/actionpack-3.2.8/lib/action_view/locale/en.yml",
...
上記ログにある、Railsの本体関連のgem経由で読み込まれてる奴のソースは以下。
- https://github.com/rails/rails/blob/master/activesupport/lib/active_support/locale/en.yml
- https://github.com/rails/rails/blob/master/activemodel/lib/active_model/locale/en.yml
- https://github.com/rails/rails/blob/master/activerecord/lib/active_record/locale/en.yml
- https://github.com/rails/rails/blob/master/actionpack/lib/action_view/locale/en.yml
Rails GuidesのRails Internationalization (I18n) API
も参照。