module I18n::Backend::Flatten

def self.normalize_flat_keys(locale, key, scope, separator)

It also handles escaping the translation keys.
and creates way less objects than the one at I18n.normalize_keys.
normalize_keys the flatten way. This method is significantly faster 
def self.normalize_flat_keys(locale, key, scope, separator)
  keys = [scope, key]
  keys.flatten!
  keys.compact!
  separator ||= I18n.default_separator
  if separator != FLATTEN_SEPARATOR
    from_str = "#{FLATTEN_SEPARATOR}#{separator}"
    to_str = "#{SEPARATOR_ESCAPE_CHAR}#{FLATTEN_SEPARATOR}"
    keys.map! { |k| k.to_s.tr from_str, to_str }
  end
  keys.join(".")
end