Following the codex about the get_header function
You must have an archive header-de.php
in your template.
Create one for each language you have and adjust the switch
accordingly.
<?php
if(function_exists('qtrans_getLanguage')) {
$lingo = qtrans_getLanguage();
switch ($lingo) {
case 'de':
get_header('de');
break;
default:
get_header();
break;
}
} else {
get_header();
}