Hello Abdussamad,
WP.org didn't notify us about the new ticket, so hence one week late reply.
To produce the problem have added this line into wp-config.php:
define('WP_DEBUG', true);
Had a looked in the code and found no place were the plugin generates any input during the activation.
Indeed, with WP_DEBUG set to true, on activation, WP shows the message:
The plugin generated 314 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.
Though the plugin is properly activated. And it is working good. Pay attention, QL menu is accessible and the plugin is set to active. You can deactivate it and activate it.
The issue you actually run into is that with WP_DEBUG on you ask WP to tell you about all PHP warnings etc...
One of those can be:
Notice: Undefined index: footer_textarea in /srv/www/clients/client1/web1/web/wp-content/plugins/quick-localization/index.php on line 61
Notice: Undefined index: only_unknown in /srv/www/clients/client1/web1/web/wp-content/plugins/quick-localization/index.php on line 65
...
These are not errors just warnings. All works well though.
With WP_DEBUG set to on you probably also get tonnes of other debug messages.
Once you set WP_DEBUG to false, the warning on activation disappears.
Let us know if you have any other issue with the plugin.
It now works on thousand of sites with no issue.