Ghost taxonomies, error messages, unecessary huge load on server

Hi,
I have an annoying problem with my Drupal 6 installation.

I have translation enabled for french. For the 1000 taxonomies and nodes on my site, I translated only one page. It has the prefix fr/ set by drupal.

When going randomly to a non translated taxonomy with the fr/ prefix, instead of loading the page not found, it will load a bogus page that generate a lots of errors.
Example: I have a taxonomy with pathauto link: /category/medium/assembly . When adding fr in front of it: fr/category/medium/assembly it will not load a page not found message, but it will load a page with lots of broken image links, generating lots of errors.

Worse, it seems search engine are visiting all the taxonomies of my site, adding the fr/ prefix in front of each taxonomy terms, genrating tons of errors. I don't know where it finds the links with the fr since they don't appear on my site. This ads unecessary load to my server.

What could be the cause of this ? Here are my modules:

XMLSITEMAP (it doesn't contain the bad links, i loaded it)
simplenews
contemplate
cck
pathauto
token
image_upload
views
ligntbox2
poormanscron
devel