You must first enable simultaneous interpretation in your Personal Room settings. Once you start a Personal Room meeting, you can then assign interpreters, and manage source and target languages.
Enable simultaneous interpretation for a Personal Room
Check Enable simultaneous interpretation in my Personal Room meetings.
Assign interpreters in a Personal Room meeting
From the Home page on your Webex Site, click Start a Meeting.
Invite people to your Personal Room. Go to More options and click Invite and Remind.
In the Personal Room meeting, click.