def cross_join(locales, keys)
      locales.flat_map do |locale|
        keys.map { |key| join(locale, key) }
      end
    end
def cross_join(locales, keys)
      locales.flat_map do |locale|
        keys.map { |key| join(locale, key) }
      end
    end
Combines each locale with all keys.

 @return [Array<String>]
 @param locales [Array<String, Symbol>]
 @param keys [Array<String, Symbol>]

 @example
   Scope.cross_join([:de, :en], [:hello, :hi]) #=>
     ["de.hello", "de.hi", "en.hello", "en.hi"]