just a note about using html codes like ü in localization files:
When using "Menü" as the message title for "Pages" everything after "Men" is cut off in the output. I suggest replacing such code like in sidebar.php with this:
<?php wp_list_pages('title_li=<h2>' . rawurlencode(__('Pages')) . '</h2>' ); ?>
When using parse_str in wp_list_pages function the "&" is treated as a seperator.
What do you think?