Caroline,
looks your acs-lang message catalog isn't imported properly.
Try create file foo.tcl with one line:
lang::catalog::import -locales [list "en_US"]
and put in under openacs-5/www directory.
Next go to address http://yourserver.com/foo and it should import missing messages.
I had the same problem, as you and this is a reason I put line
lang::catalog::import -locales [list "en_US"]
into Dave's zzz-postload.tcl file
