Go to the page: administrator/index.php?option=com_installer&view=languages

Click on the button "Find Languages"

The page may remain empty on a manually updated site to last staging, even after database fix.

This is due to this update sql: ROOT/administrator/components/com_admin/sql/updates/mysql/3.6.0-2016-04-08.sql

INSERT INTO `#__extensions` (`extension_id`, `name`, `type`, `element`, `folder`, `client_id`, `enabled`, `access`, `protected`, `manifest_cache`, `params`, `custom_data`, `system_data`, `checked_out`, `checked_out_time`, `ordering`, `state`)
 VALUES (801, 'English (United Kingdom)', 'package', 'pkg_en-GB', '', 0, 1, 1, 1, '', '', '', '', 0, '0000-00-00 00:00:00', 0, 0);

-- Change update site extension id to the new extension.
UPDATE `#__update_sites_extensions` SET `extension_id` = 801 WHERE `update_site_id` = 3 AND `extension_id` = 600;

On one of my sites the id 801 was used by the weblinks package:

I suggest to NOT at least change the id 600 to 801 in _update_sites_extensions