<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def pages_sub_menu(context, page, url='/'):
 """Get the root page of the given page and render a nested list of all root's children pages. Good for rendering a secondary menu. :param page: the page where to start the menu from. :param url: not used anymore. """

    lang = context.get('lang', pages_settings.PAGE_DEFAULT_LANGUAGE)
    page = get_page_from_string_or_id(page, lang)
    if page:
        root = page.get_root()
        children = root.get_children_for_frontend()
        context.update({'children': children, 'page': page})
    return context