How to Change the language used at the login screen on your Mac

The login screen is displayed in the language you selected the first time you set up your Mac. You can change it using these steps.

If there’s only one user account on your Mac

If there's only one user account on your Mac, use these steps to change the primary language you use with that account. This also changes the language used at the login screen:

1. Choose System Preferences from the Apple menu.
2. Click Language & Region.
3. Drag the desired language to the top of the Preferred languages list. If the language you want isn't listed, click the Add (+) button to add the language you want.
4. Restart your Mac.

After your Mac restarts, the login screen uses the language you set.

If there’s more than one user account on your Mac

If there's more than one user account on your Mac, or you want to change only the language at the login screen, use these steps:

1. Log in as an admin user.
2. Open Terminal from the Utilities folder.
3. Type or paste this command in Terminal:

   sudo languagesetup
   

4. Press Return.
5. When prompted for a password, enter an administrator account password and press Return.
   
6. A list of available languages is displayed. Type the number that appears next to the language you want to use, then press Return.
7. Quit Terminal.

The next time you log out or restart your Mac, the login screen uses the language you set.

Change your keyboard layout

To select a different input method or keyboard at the login screen, click the Input menu in the upper-right corner of the menu bar and choose an option.

If you don't see the Input menu, use these steps to add it to the login screen:

1. Log in.
2. From the Apple menu, choose System Preferences.
3. Click the Users & Groups icon.
4. Click the lock  icon. Enter an admin user name and password.
5. Click Login Options.
6. Select the option to Show Input menu in login window.

Source: https://support.apple.com/en-us/HT202036