When writing your wddx file manually with an UTF-8 aware editor and saving it in utf-8, if your data gets its special characters mysteriously scrambled, try to add an xml header that marks the output as iso-8859-1, like this one:
<?xml version="1.1" encoding="iso-8859-1" ?>
This makes the wddx decode function treat the input as iso-8859-1, so it will not try to treat it as utf-8 and do an implicit decode to iso-8859-1. You will then have read all string data in the wddx packet in their original utf-8 encoding, so that 'echo' and other output functions will produce the intended result if you have set the output encoding to utf-8.
(Bugs reports on this behaviour seem to be treated as bogus, so it would seem in order to point out this incorrect and highly confusing side-effect.)