up to now, it is not possible to set the FirstSpirit master language as a mandatory language. Currently, there is no configuration switch available. Details see the following screenshot showing the FirstSpirit AdminClient language setup enclosed. It just offers the possibility to configure the master language.
The need to set the master language as a mandatory language has its origin in a proper and save FirstSpirit link generation. Imagine, the link target would not be edited in the master language. So, it might be possible to not activate any language as "completely translated" - even not the master language. But the page reference of this page is still selectable as link target by the editor, although this page is not marked as translated.
The consequence of this feature request: the master language that is configured as mandatory might has to be marked as fully translated by the editor.