I believe the best choice could be Java.
Is very flexible, easy to use, run on a huge list of OS's including Linux and Windows, and have support for SOAP.
This will be my choice : Java, but you can write on any language.
Perl and Python and PHP and Script Languages should be my last choice; because you are writing a "server" application.
If you choose java, you can found all the information you need and even more!, in http://www.sun.com/java
site. Look for "The Java Tutorial" for java programming and "The J2EE Tutorial" for SOAP, XML and Web Services info.
Background: I know programming in all those, Python not very much.