As an Administrator, you can set the User's default language at the time of creating a User's account or when viewing the User's profile once it has been created.
When creating a User account
Complete the fields within the first step of the New User set-up wizard and select the language from the Language dropdown menu at the bottom of the form before clicking on the Next button.
When viewing a User's profile
Locate the User's profile within the Organisations and User's area. Click on their Username to view their profile.
Click on the My Preferences tab from the left-hand menu then select the User's default language from the Language dropdown menu.
Click on the Save Changes button.