Now jBPM 4 could support JUEL, beanshell and groovy, If you want to use python in jpdl.xml, you should create a new ScriptEngineFactory, may be PythonScriptEngineFactory and put it into <script-manager>.
The default <script-manager> configuration is as following:
<?xml version="1.0" encoding="UTF-8"?>
<jbpm-configuration>
<process-engine-context>
<script-manager default-expression-language="juel"
default-script-language="juel">
<script-language name="juel" factory="org.jbpm.pvm.internal.script.JuelScriptEngineFactory" />
<script-language name="bsh" factory="org.jbpm.pvm.internal.script.BshScriptEngineFactory" />
<script-language name="groovy" factory="org.jbpm.pvm.internal.script.GroovyScriptEngineFactory" />
</script-manager>
</process-engine-context>
</jbpm-configuration>