Try not to separate the message tag and CDATA portion. The parser might create a text node enclosing the CDATA if it sees any space, and xwork just reads the first node (therefore, an empty text). Example:-
<field name="password">
<field-validator type="requiredstring">
<message><![CDATA[ Debe proporcionar una contraseña para el usuario. ]]></message>
</field-validator>
</field>
Note: there's no spacing in the following line
<message><![CDATA[ ...]]></message>
Contributor: Ruben