How can I allow the user to choose their language upon registration?
As Drupal 7 doesn't show the language selector for the users who create their own account, you need to implement hook_form_user_register_form_alter() (see the documentation for hook_form_FORM_ID_alter()) using the following code.

function mymodule_form_user_register_form_alter(&amp;$form, &amp;$form_state) {
  // Only alter user forms if there is more than one language.
  if (!isset($form['locale']) &amp;&amp; drupal_multilingual()) {
    locale_language_selector_form($form, $form_state, $form['#user']);
  }

  if (isset($form['locale']) { 
    $form['locale']['#access'] = TRUE;
  }
}


The code is generic, and works even in the case a module removed the language selector field (which is something a module should never do, though).

"user_register_form" is the form builder used for the "user/register" path. The code works if the Locale module is enabled. The code will show the language selector if there are more than one language enabled; differently, drupal_multilingual() would return FALSE. 

  $items['user/register'] = array(
    'title' =&gt; 'Create new account', 
    'page callback' =&gt; 'drupal_get_form', 
    'page arguments' =&gt; array('user_register_form'), 
    'access callback' =&gt; 'user_register_access', 
    'type' =&gt; MENU_LOCAL_TASK,
  );


The code used from the Locale module is the following one.

function locale_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  // Only alter user forms if there is more than one language.
  if (drupal_multilingual()) {
    // Display language selector when either creating a user on the admin
    // interface or editing a user account.
    if ($form_id == 'user_register_form' || ($form_id == 'user_profile_form' &amp;&amp; $form['#user_category'] == 'account')) {
      locale_language_selector_form($form, $form_state, $form['#user']);
    }
  }
}


&nbsp;

In general, the equivalent for hook_user('register') in Drupal 7 is hook_form_user_register_form_alter(); the equivalent of hook_user('form') is hook_form_user_profile_form_alter().