What encoding are you currently using in your language file? This sounds pretty much like what I encountered when I was testing utf-8 for the encoding.
For reference, I'm just using an English-language board with all default settings, and I haven't touched any encodings.
For me the utf8_encode() tweak in quickquote.inc.php fixed my problems with the preview. The "QUICK QUOTE" button also works; I have no idea if it used to work before or not. I also have no idea why editing quickquote.inc.php (not previewpost.inc.php) fixed previewing...
