Umlauts for German WordPress slug - add these lines to your functions.php and forget it.

dnb-transliterate-slug.php

<?php

add_filter('sanitize_title', 'dnb_transliterate_slug', 5, 3);
function dnb_transliterate_slug($title, $raw_title = NULL, $context = 'query') {
    // Hacky hook due to hacky core, see
    // http://core.trac.wordpress.org/ticket/16905

    if ($raw_title != NULL) {
        $title = $raw_title; // undo remove_accents
    }

    $title = str_replace('Ä', 'ae', $title);
    $title = str_replace('ä', 'ae', $title);
    $title = str_replace('Ö', 'oe', $title);
    $title = str_replace('ö', 'oe', $title);
    $title = str_replace('Ü', 'ue', $title);
    $title = str_replace('ü', 'ue', $title);
    $title = str_replace('ẞ', 'ss', $title);
    $title = str_replace('ß', 'ss', $title);

    if ($context == 'save') {
        $title = remove_accents($title); // redo remove_accents
    }

    return $title;
}