I've already edited the function linkUrls to add the german umlauts ÄÖO:
$text = preg_replace_callback("/(^|\s)(([a-zA-Z]+:\/\/)([a-z][a-z0-9_\..-]*[a-z]{2,6})([a-zA-Z0-9äöüÄÖÜ~\/*-?&%]*))/i", array($this, '_linkUrlsCallback'), $text);
How do I add the same thing to the linked hash tags in the function linkHashtags?