add_filter( 'gettext', 'change_admin_menu_pages' );
add_filter( 'ngettext', 'change_admin_menu_pages' );
function change_admin_menu_pages( $translated ) {
    $translated = str_replace( 'Page', 'Core Page', $translated );
    $translated = str_replace( 'page', 'core page', $translated );
    return $translated;
}

add_action('admin_menu','rename_pages_to_main_sections');
function rename_pages_to_main_sections() {
  global $menu;
  global $submenu;
  $menu[20][0] = 'Main Sections';
  $submenu['edit.php?post_type=page'][5][0] = 'Main Sections';
}