You placed something like this in config/application.rb
I18n.enforce_available_locales = true
config.i18
The reason is due to a bug with locales that have the language-country format. The good news is that it has been fixed already. This was reported in Github with Issue 13164 and fixed with pull request 229.
If you want to grab the latest code with the fix, update the gem like this:
Update your Gemfile with:
gem 'i18n', github: 'svenfuchs/i18n'
And then: bundle install