How do I customise individual menu links?
If you just need to add a string after the menu title, you can use the following code.

function mytheme_menu_link(array $variables) {
  $element = $variables['element'];
  $sub_menu = '';

  if ($element['#below']) {
    $sub_menu = drupal_render($element['#below']);
  }

  $output = l($element['#title'] . ' ' . t('the string you need to add'), $element['#href'], $element['#localized_options']);

  return '&lt;li' . drupal_attributes($element['#attributes']) . '&gt;' . $output . $sub_menu . "&lt;/li&gt;\n";
}


The alternative would be using the following code, which would give to who translate the string more context to translate it.

function mytheme_menu_link(array $variables) {
  $element = $variables['element'];
  $sub_menu = '';

  if ($element['#below']) {
    $sub_menu = drupal_render($element['#below']);
  }

  $output = l(t('!title the string you need to add', array('!title' =&gt; $element['#title'])), $element['#href'], $element['#localized_options']);

  return '&lt;li' . drupal_attributes($element['#attributes']) . '&gt;' . $output . $sub_menu . "&lt;/li&gt;\n";
}


You could also implement a preprocess function to alter the title.

function mytheme_preprocess_menu_link(array $variables) {
  $variables['element']['#title'] = t('!title the string you need to add', array('!title' =&gt; $variables['element']['#title']));
}