XML defines only a few entities, so if you want to use € you have have a definition for it in a DTD. Tony Coates and Zarella Rendon propose a non-DTD way around the problem for entities in text nodes using an XSLT transform library.
This is the first entry in a new category, I18N. I’ve started tracking this since Internationalizaton and Localization are a big part of this year’s projects.
