1. Open the Administrative console (e.g. https://localhost:9043/ibm/console/logon.jsp)
2. Expand System administration
3. Click Deployment manager
4. Expand Java and Process Management
5. Click Process Definition
6. Click Java Virtual Machine
7. As Generic JVM arguments, add the JVM argument(s), e.g. -Duser.language=en
8. Click OK
9. Click Save
10. Restart the server