Minimal Rails 4.0.2 application that reproduces an issue where the I18n::Fallbacks module will raise spuriours I18n::InvalidLocale errors where translating an attribute name that does not exist in the translation file.

config_application.rb

require File.expand_path('../boot', __FILE__)

require 'rails/all'

# Require the gems listed in Gemfile, including any gems
# you've limited to :test, :development, or :production.
Bundler.require(:default, Rails.env)

module A
  class Application < Rails::Application
    config.i18n.default_locale = "pt-BR"
    I18n.enforce_available_locales = true
  end
end

config_locales_pt-BR.yml

pt-BR:
  hello: "Olá mundo"