How do I use the geoip_language_redirect module?
As the project page says, after you installed the module, you need to set up the geoip_redirect_mapping persistent variable with an array that associate the ISO country code with the language.

variable_set('geoip_redirect_mapping', array(
  'GB' =&gt; 'en',
  'FR' =&gt; 'fr',
  // …
));


For example, put the following code in setup.php, and save it in the same directory where Drupal has its cron.php and index.php.

define('DRUPAL_ROOT', getcwd());

require_once DRUPAL_ROOT . '/includes/bootstrap.inc';
drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);
variable_set('geoip_redirect_mapping', array(
  'GB' =&gt; 'en',
  'FR' =&gt; 'fr',
  // …
));


Visit http://example.com/setup.php, and the variable will be setup. After this, you can delete the file.

If you have Drush, then you can alternatively execute the following command from the site directory (from the terminal).

php -r "print json_encode(array('GB' =&gt; 'en', 'FR' =&gt; 'fr'));"  | drush vset --format=json geoip_redirect_mapping -


For the rest, the module automatically redirects the users.

Please note the following

The project page is a little confusing. It's not clear if the persistent variable is geoip_mapping or geoip_redirect_mapping. I didn't find in the code any reference to that variable, so I cannot confirm which one is correct. I would try geoip_redirect_mapping, since that is the one used from the example code; if that doesn't work, I would try geoip_mapping. If neither of them works, I would open a support request for the module, if there isn't one already, asking to change the project page to make it clearer.