For Admins:
Navigation: Settings > Global Settings > User Settings
As a site admin, you can change the default language for the entire organization.
For Employees:
Navigation: Profile Picture > Profile > Edit Profile > Preferences
As an employee, you may set your own preferred language. However, an admin can enable/disable this permission for users at any time.
Note: Changing the default language as a site admin (from the Settings menu) will reset the users' preferred languages.