Select the language at login

Just add a callback for the login screen:

add_action( 'login_form', function()
{
    // do stuff here
} );

or, if you have to use PHP 5.2:

add_action( 'login_form', 'your_additional_element' );
function your_additional_element()
{
    // do stuff here
}

Then just look at the plugins source code and add an additional element. The plugins functions all are public. Make sure to save the data afterwards. In the plugin we use a one time running callback on the locale hook.