You should use the decimal character references for these characters to use them in your XML. So instead of writing รก, you should write '& # 2 2 5 ;' (remove the spaces). This way Flash will display the characters correctly.
A list of the char references can be found at: http://www.bbsinc.com/iso8859.html
EDIT: Another way of solving this is setting the XML encoding to UTF-8.
XML:
-
<?xml version="1.0" encoding="utf-8"?>