Each file has as series of <resource> elements each with an xml:lang attribute. You need to add your own element. Say you want to localize to Italian:
You can start by copying an existing <resource> element (for example French).
Starting 2014-03-21, we are marking placeholder resources for existing languages with todo="true".
NOTE: You must not translate the <value> part of item definitions, for example:
<hint>Se è necessario immettere i dati</hint>
For Form Builder, starting with Orbeon Forms 4.0, you also need to update the following property in your properties-local.xml:
value="en fr it"/>
By default, Orbeon Forms 4.0 sets it to en fr. The updated property at least needs to include the new language or languages that you are adding, in this case, it. For more details on this, see this Stack Overflow question.