It is possible using the template_redirect
hook.
Looks something like this :
function language_redirect()
{
global $q_config;
if( $q_config['lang'] == 'en' )
{
include( get_template_directory() . '/page-template_en.php' );
exit;
}
else
{
include( get_template_directory() . '/page-template_de.php' );
exit;
}
}
add_action( 'template_redirect', 'language_redirect' );
Code is untested, but should look like that.
See my similar answer HERE for more help.