module I18n::Backend::Fallbacks

def translate(locale, key, options = EMPTY_HASH)

it is evaluated last after all the fallback locales have been tried.
it's a Symbol. When the default contains a String, Proc or Hash
The default option takes precedence over fallback locales only when

locales it will then throw MissingTranslation as usual.
the given options. If it does not find any result for any of the
(depends on the fallbacks implementation) until it finds a result with
locale :"de-DE" it might try the locales :"de-DE", :de and :en
locale given by I18n.fallbacks for the given locale. E.g. for the
Overwrites the Base backend translate method so that it will try each 
def translate(locale, key, options = EMPTY_HASH)
  return super unless options.fetch(:fallback, true)
  return super if options[:fallback_in_progress]
  default = extract_non_symbol_default!(options) if options[:default]
  fallback_options = options.merge(:fallback_in_progress => true, fallback_original_locale: locale)
  I18n.fallbacks[locale].each do |fallback|
    begin
      catch(:exception) do
        result = super(fallback, key, fallback_options)
        unless result.nil?
          on_fallback(locale, fallback, key, options) if locale.to_s != fallback.to_s
          return result
        end
      end
    rescue I18n::InvalidLocale
      # we do nothing when the locale is invalid, as this is a fallback anyways.
    end
  end
  return if options.key?(:default) && options[:default].nil?
  return super(locale, nil, options.merge(:default => default)) if default
  throw(:exception, I18n::MissingTranslation.new(locale, key, options))
end