def default_locale_negotiator(request):
    """ The default :term:`locale negotiator`.  Returns a locale name
    or ``None``.

    - First, the negotiator looks for the ``_LOCALE_`` attribute of
      the request object (possibly set by a view or a listener for an
      :term:`event`). If the attribute exists and it is not ``None``,
      its value will be used.

    - Then it looks for the ``request.params['_LOCALE_']`` value.

    - Then it looks for the ``request.cookies['_LOCALE_']`` value.

    - Finally, the negotiator returns ``None`` if the locale could not
      be determined via any of the previous checks (when a locale
      negotiator returns ``None``, it signifies that the
      :term:`default locale name` should be used.)
    """
    name = '_LOCALE_'
    locale_name = getattr(request, name, None)
    if locale_name is None:
        locale_name = request.params.get(name)
        if locale_name is None:
            locale_name = request.cookies.get(name)
    return locale_name
