raise exception on missing translations in test and dev environment

config/initializers/locale.rb

@@ -1,2 +1,8 @@
 I18n.load_path += Dir[Rails.root.join('config', 'locales', '*.{rb,yml}')]
+
+if Rails.env.development? || Rails.env.test?
+  I18n.exception_handler = lambda do |exception, locale, key, options|
+    raise "Missing translation: #{key}"
+  end
+end