By default the character encoding is UTF-8. However, this can be changed by using the new encoding selection in Designer 8 or by modifying the encoding property within the XML of the PDF.
I've uploaded a sample for you here:
http://my.adobe.acrobat.com/barcodedformssampleharacterencodingpdf417/In the XML, the barcode element would look something like this:
Just be aware that some hand scanners do not work well with double-byte character set encoding so you may want to perform your tests with a software based decoder to avoid problems.