Cart block disappears when I switch default language to french !

Cart block disappears when I switch default language to french, and it comes back when I switch back to english ... unfortunately the site I'm currently working on HAS to be in french by default.
Cart page view (/cart) works fine in any language ... only the block disapears (I checked the block config and saw nothing unusual).
No error message, nothing in logs (neither in drupal's db log nor in apache/php log file).
It's been two days since I'm working on it, this is driving me crazy !!!
Any idea about what may cause this ?
Any tip to track the origin of that bug ?
Please, help me !

Thanks,

Hervé