The countries are collected in the user view file (views/user/view.html.php )
$lists["country"] = AdsUtilities::makeCountryList($user->country,null,350);
The method AdsUtilities::makeCountryList is in the helpers/helper.php file.

I don't know what you might want to tweak in here, but if you want to alter the helper method somehow , I suggest you create your personalized method (my_makeCountryList ) to do what you intend to do.

Cheers,
Sorin

In this case just replace the
  $lists["country"] = AdsUtilities::makeCountryList($user->country,null,350);
from the view with
  $lists["country"] = " <input type='hidden' name='country' value ='76' />"
 ( 76 would be the id of France  )

Sorin