template.php breadcrumb function
Breadcrumb is made up of an array. So to add any element in the breadcrumb you have to give the offset.

So if you want to set t('Du er her') in place of the front page link then you should try this:

$breadcrumb[0] = l(t('Du er her'), 'front');


To know the exact position of element which do you want to add, you can use var_dump($breadcrumb).

I would rewrite the code as follows.

function modeindeks_breadcrumb($variables) {
  $breadcrumb = $variables['breadcrumb'];  

  if (!empty($breadcrumb)) {
    // Provide a navigational heading to give context for breadcrumb links to
    // screen-reader users. Make the heading invisible with .element-invisible.
    // Instead of front, you can use the URL of your site's front page.
    $breadcrumb[0] = l(t('Du er her'), 'front'); 

    $output .= '&lt;div class="breadcrumb"&gt;' . implode(' › ', $breadcrumb) . '&lt;/div&gt;';

    return $output;
  }
}


If you want to add only You are here link then use the below code.

function modeindeks_breadcrumb($breadcrumb) {
  if (!empty($breadcrumb)) {
    return '&lt;div class="breadcrumb"&gt;You are here: ' . implode(' › ', $breadcrumb) . '&lt;/div&gt;';
  }
}