Jhon,
Are you using a Spanish language install of Service Desk, or is your copy a heavily customized English language install?
The concern that I have is that if you were to modify the pdm.xml file directory (which contains the message) it's very likely the file will be overwritten if you were to apply a cumulative fix to your system in the future. I checked in the version of pdm.xml provided in the Spanish patch and see:
<msg id="03121" ><text product_id="default">Falta el atributo %1 dependiente del objeto %2. Cancele y rellene el campo %1.</text><action>Supply a value for the specified attribute.</action><reason> "You have attempted to create or update a record of type object-name, but are missing a required dependent attribute. The object-name and attr-name in the message are not necessarily the same as the labels on the form. If necessary, consult the Reference Guide for a description of all objects and attributes."</reason></msg>
Are you seeing this, or does it appear as English for you?