If you use spawn="true", this is not possible. Set spawn to false (default), and it works.
Update:
Try using:
<java fork="true"
jar="${languagesynch.path}/dist/test.jar">
<arg value="C:\ContinuousIntegration\res" /> <!--copy from-->
<arg value="${project.basedir}\res" /> <!--to this directory-->
</java>
Or if for any reason you MUST use exec:
<exec executable="cmd.exe" spawn="false">
<arg value="/c"/>
<arg value="java"/>
<arg value="-jar"/>
<arg value="${languagesynch.path}"/>
<arg value="C:\ContinuousIntegration\res" /> <!--copy from-->
<arg value="${project.basedir}\res" /> <!--to this directory-->
</exec>