Fatal errors like

  % HEAD -H "Accept-Language: de" \
     http://jigsaw.w3.org/css-validator/validator

  Cache-Control: no-cache
  Connection: close
  Date: Sat, 29 May 2004 02:20:57 GMT
  Pragma: no-cache
  Server: Jigsaw/2.2.2 W3C_CSS_Validator_JFouffa/2.0
  Content-Language: en
  Content-Length: 918
  Content-Type: text/html; charset=us-ascii
  ...

always declare us-ascii encoding even though localized error messages as the 
Validator would use in this example might not be us-ascii encoded. It might be 
possible to extract the proper charset from the property files, but that does 
not work currently, see Bug #162. It might be best to switch to UTF-8 for all 
output.

The responsible code here is org.w3c.css.servlet.CssValidator.handleError().