How can I change English for an alternative language on iMac Pro after installing macOS Sonoma 14.7.5?

You're welcome. 🙂

14.7.5 is Sonoma, not Sequoia. It isn't possible for your iMac to be 2019 model as the very first M1 Mac wasn't released until November 17, 2020.

It's normal to set up the first new user's name and password. Someone needs to be the admin user on a Mac when no other user accounts yet exist.

I've never heard of the initial setup using more than one language. One of very first questions, even before setting up the initial user account, is what language you want to use. You can only choose one.

Since nothing else is on the Mac, it would be easiest to start over in Recovery Mode.

Shut the Mac down. Then press and hold the power button until you get see the Options button. Choose that and then Continue.

At this point, the normal method is to click on the admin user name and then enter the password. But, that would be the same password it already isn't accepting. So, do this instead:

1 ) Click on the link, Forgot all passwords?

2) You'll get this message:

This Mac is linked to an Apple Account. Sign in to the account that was previously used with this Mac.

3) A partial of the email address (your Apple ID) will be shown. Type in the full address in the box underneath. It will then prompt you for the account password.

4) Assuming you've entered the information correctly, you'll get a message that your entries have been verified. A link to Exit to recovery will appear.

Now you can use Disk Utility to erase the drive and start over with fresh install of the OS. Or, supposedly, you can simply choose to reinstall the OS (without the erase step) and you'll be asked to create a new admin account when it gets that far.

If you continue to experience issues, you might want to reset the NVRAM (non-volatile random access memory). This can sometimes resolve login issues related to user accounts. To do this, restart your Mac and hold down Option + Command + P + R for about 20 seconds.