Validate all nested "language" key in `obj`.

       Args:
           obj (dict): dictionary whose "language" key is to be validated.

       Returns:
           None: validation successful

        Raises:
            ValidationError: will raise exception in case language is not valid.

def validate_language_key(obj, key):
    """Validate all nested "language" key in `obj`.

       Args:
           obj (dict): dictionary whose "language" key is to be validated.

       Returns:
           None: validation successful

        Raises:
            ValidationError: will raise exception in case language is not valid.
    """
    backend = bigchaindb.config['database']['backend']

    if backend == 'localmongodb':
        data = obj.get(key, {})
        if isinstance(data, dict):
            validate_all_values_for_key_in_obj(data, 'language', validate_language)
        elif isinstance(data, list):
            validate_all_values_for_key_in_list(data, 'language', validate_language)