Here's a simple example, it needs to be extended with locale check to make sure we don't save invalid locales.

// Detect visitor's locale
if( ! $Session->has_User() )
{
	if( $locale = param('locale', 'string') )
	{
		if( $locale == 'reset' )
		{	// Reset saved locale
			$Session->delete('manual_locale');
		}
		else
		{	// Save selected visitor's locale for this session
			$Session->set('manual_locale', $locale);
		}
	}
	// Activate saved locale
	locale_activate( $Session->get('manual_locale') );
}