For a multi-site/tenant setup, one master language is defined. Could there possibly be more than one master language since you are dealing with multiple sites?
This question comes up from time to time, but support for this is unavailable at the moment.
Unfortunately, support for multiple master languages for multi-tenant/sites is not available. Forum thread example: http://world.episerver.com/forum/developer-forum/-Episerver-75-CMS/Thread-Container/2016/2/episerver-9-multi-site-different-master-lanugages/