If Implementation is set to 'Java' in BusinessRule task by JBDS/BPMN2 Modeler like the following,
~~~
<bpmn2:businessRuleTask id="BusinessRuleTask_1" tns:ruleFlowGroup="Hello" name="Business Rule Task 1" implementation="Java">
<bpmn2:extensionElements>
<tns:metaData name="elementname">
<tns:metaValue><![CDATA[Business Rule Task 1]]></tns:metaValue>
</tns:metaData>
</bpmn2:extensionElements>
<bpmn2:incoming>_1-_2</bpmn2:incoming>
<bpmn2:outgoing>SequenceFlow_1</bpmn2:outgoing>
</bpmn2:businessRuleTask>
~~~

rule flow validation fails like the following
~~~
05:33:59.750 [main] ERROR o.d.c.kie.builder.impl.KieProject - Unable to build KieBaseModel:process
[-1,-1]: Process 'Hello World' [com.sample.bpmn.hello]: Node 'Business Rule Task 1' [3] Unsupported rule language 'Java'

java.lang.RuntimeException: Error while creating KieBase[Message [id=1, kieBase=process, level=ERROR, path=/Users/okuniyas/dev/workspace/RuleFlow test/target/classes/process/sample.bpmn, line=-1, column=0
text=Process 'Hello World' [com.sample.bpmn.hello]: Node 'Business Rule Task 1' [3] Unsupported rule language 'Java']]
at org.drools.compiler.kie.builder.impl.KieContainerImpl.getKieBase(KieContainerImpl.java:554)
at org.drools.compiler.kie.builder.impl.KieContainerImpl.newKieSession(KieContainerImpl.java:679)
at org.drools.compiler.kie.builder.impl.KieContainerImpl.newKieSession(KieContainerImpl.java:647)
at com.sample.ProcessTest.main(ProcessTest.java:17)
~~~

This worked with BRMS6 but not with RHDM7.
It seems that this attribute is strictly checked due to DMN support.

In BPMN2 Modeler, the following 4 options can be selected as implementation.
Java
SwitchYard Service
Unspecified
WebService

But only latter 2 are allowed in BusinessRuleTask.

Since implementation can't be set in BusinessRuleTask by Process Designer of Decision Central/Business Central, BPMS2 Modeler should behave same in order to avoid above error.