Solution

1. Back up the site and the database.
2. Go to Persona Bar > Settings > SQL Console.
3. Delete the culture code of the aborted localization process.
   Run the following SQL script:

   
   DELETE
   FROM Tabs
   WHERE CultureCode = 'xx-XX';
                               

   where xx-XX is the culture code of the added language.
4. Disable content localization for the site.
   Run the following SQL script:

   
   UPDATE Tabs
   SET CultureCode = NULL;
   
   UPDATE PortalSettings
   SET SettingValue = 'False'
   WHERE SettingName = 'ContentLocalizationEnabled'
   	AND PortalID = xx;
                               

   where xx is the ID of the site for which localization was enabled.

5. Recycle the application pool to allow the changes to take effect.

6. Rerun the localization process.