<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def default_config_filename(root_dir=None):
 """ Returns the default name of the configuration file. """

        root_dir = Path(root_dir) if root_dir else Path('.').abspath()
        locale_dir = root_dir / 'locale'
        if not os.path.exists(locale_dir):
            locale_dir = root_dir / 'conf' / 'locale'
        return locale_dir / BASE_CONFIG_FILENAME