How to set rel="alternate" hreflang="x"?
You tagged your question as Drupal 7 so I'm guessing the only problem is that you're using drupal_set_html_head() which doesn't exist in Drupal 7.

It's been replaced with drupal_add_html_head(), which you can use like so:

$element = array(
  '#tag' =&gt; 'link', 
  '#attributes' =&gt; array( 
    'hreflang' =&gt; 'some_language', 
    'rel' =&gt; 'alternate',
  ),
);
drupal_add_html_head($element, 'mymodule');