Something is breaking JoomlaComment. I can't figure out what it is, all I can tell is that the Comment form code in Firebug/Source is all $@#&ed up in astronomically outrageous ways!
Behold the following Code block from your page source nico -
<a href='javascript:JOSC_insertUBBTag("u")'><img src='http://culturejapon.fr/components/com_comment/joscomment/images/ubb_underline.gif' class='buttonBB' name='bu' alt='<span style="text-decoration:underline;">' /></a>
<a href='javascript:JOSC_insertUBBTag("url")'><img src='http://culturejapon.fr/components/com_comment/joscomment/images/ubb_url.gif' class='buttonBB' name='burl' alt='<a target='_blank'>' /></a>
...don't see it? The first one, underline, has alt='<span style="text-decoration:underline;" at the end. What the!?!? Then the alternate text for the second one, URL, alt='<a target='_blank'>'
Something strange is going on. Where is that comming from? If that was happening to me, I wouldn't know how to start diagnosing it. All I can suggest is to disable one extension at a time, going backwards to using original Joomla purity template and only JoomlaComment - process of elimination would at least pin-down the cause of the phantom code. It's probably the Joomla template, but it's really strange.
Of course you can't shut down your site though. Well maybe you can, but maybe Daniel can find something out without you having to do that hopefully.