Right-to-Left (RTL) Support has finally been added to Office for Mac 2016 in v15.20. Currently v15.20 is only available to Office Insiders. If you can’t wait for the official release, opt in to join Office Insider.
Office Insider builds for Mac are now available, you can opt in through Microsoft AutoUpdate. To access AutoUpdate, start an Office 2016 for Mac program, and then select Check for Updates on the Help menu.
For more info on Office Insider see: Be an Office Insider
For support visit the Microsoft Forum Office Insider