def _add_extension_assets(client, customer_id, language_code):
    """Creates new assets for the hotel callout.

    Args:
        client: an initialized GoogleAdsClient instance.
        customer_id: a client customer ID.
        language_code: the language of the hotel callout feed item text.

    Returns:
        a list of asset resource names.
    """
    operations = []
    # Create a hotel callout asset operation for each of the below texts.
    for text in ["Activities", "Facilities"]:
        operation = client.get_type("AssetOperation")
        asset = operation.create
        asset.hotel_callout_asset.text = text
        asset.hotel_callout_asset.language_code = language_code
        operations.append(operation)

    asset_service = client.get_service("AssetService")
    # Issues the create request to create the assets.
    response = asset_service.mutate_assets(
        customer_id=customer_id, operations=operations
    )
    resource_names = [result.resource_name for result in response.results]

    # Prints information about the result.
    for resource_name in resource_names:
        print(
            "Created hotel callout asset with resource name "
            f"'{resource_name}'."
        )

    return resource_names