The fieldnames that appear in the errors from the validation-class aren't translated at the moment. You get the name of the databasefield.
For example a dutch user will get an english name in the error and that can be confusing.
I changed the validationclass in a few places in this way:
$translated_fieldname=getLanguageText($fieldname);
$this->errors[$fieldname] = getLanguageText('sys0021', $translated_fieldname, $size);
Now I can put the translations of the fieldnames in the language file and the problem is solved.
Maybe something to integrate in the next release? Or is there a better solution?