Chouby on "[Plugin: Polylang] db data migration when upgrading from version 1.1.x to 1.5.4 (or 1.2 and above)"

is it possible to execute migration process manually?

That would be very difficult.

Did you restore the database to 1.1.6 state and checked that everything worked ok before attempting to uprade from 1.1.6 to a newer version? Because the DB upgrade can happen only once. So reverting from 1.2+ to 1.1.6 and then upgrading to 1.2+ does not work if the database has not bee restored.

Also can you put this in your wp-config.php:

define('WP_DEBUG', true);
define('SCRIPT_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', false);

This will enable logging errors to wp-content/debug.log, so you will be able to detect if an error occurs during the upgrade.