Impossible to add the language

I have installed the plugin, but when i am adding languages its giving a error Impossible to add the language on my website https://www.sufferwell.org;

whereas i used the same plugin for my dev instance (https://www.dev.sufferwell.org) of the same website on the same server and its working fine, I tried disabling al the other plugins, changing the themes and also repairing the tables of the database but not getting any success.

Any help would be appreciated

The page I need help with: [log in to see the link]