ZK's file upload facility is based on Apache Commons FileUpload library. The error message produced by this library is reported to the client-side verbatim. However, the messages are English only.
To localize the messages, one trick is to override the message on the client-side (e.g. http://forum.zkoss.org/question/71121/customized-error-message-for-to-big-file-uploads/). Another way is to override the exception handling in AuUploader.service().