I18N permalinks (converting common accentuated UTF-8 characters automatically with WordPress)

functions.php

/**
 * I18N permalinks (Czech, German)
 */

add_filter('sanitize_title', 'transliterate_slug', 5, 3);

function transliterate_slug($title, $raw_title = NULL, $context = 'query') {

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

    // Czech
    $title = str_replace('Č', 'c', $title);
    $title = str_replace('č', 'c', $title);
    $title = str_replace('Ď', 'd', $title);    
    $title = str_replace('ď', 'd', $title);
    $title = str_replace('Ě', 'e', $title);    
    $title = str_replace('ě', 'e', $title);
    $title = str_replace('Ň', 'n', $title);    
    $title = str_replace('ň', 'n', $title);
    $title = str_replace('Ř', 'r', $title);    
    $title = str_replace('ř', 'r', $title);    
    $title = str_replace('Š', 's', $title);    
    $title = str_replace('š', 's', $title);
    $title = str_replace('Ť', 't', $title);    
    $title = str_replace('ť', 't', $title);
    $title = str_replace('Ů', 'u', $title);    
    $title = str_replace('ů', 'u', $title);
    $title = str_replace('Ž', 'z', $title);    
    $title = str_replace('ž', 'z', $title);
            
    // German
    $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);
    }

    return $title;
}