There are two options for changing default language on your WordPress Multisite install:
1. Settings in the Network Admin dashboard
Used to change the default language network wide i.e. the language that is displayed inside the dashboards of all sites network wide and on the login screen.
For example, here is what it would look like if its default language was set to Danish.
2. Settings > General in the Site Admin dashboard
Used by your users to override your default language setting. This enables your users to set their own preferred default language.