How to change the language at initial set up screen, fresh OS install?

Macs are built for the countries in which they are sold. In other words a Mac built for sale in Brazil will have a different marketing code than one built for sale in Asia. Reasons for that include local regulatory requirements—energy efficiency and Wi-Fi differences, among them.

The way you bought a Mac is the way you must sell it. It is incumbent upon a potential buyer of a used Mac to perform due diligence and determine if it will even work in their country. Let them do that. Don't do it for them, lest he or she come to this site and complain it's not working.

I have just freshly installed Mojave on a MacBook Air ...

The solution to your problem is not to do that.

When selling a Mac, follow these instructions: What to do before you sell, give away, or trade in your Mac ...

The very first thing its new owner will see is a language selection screen.

I am not sure if the system will be in portuguese once logged in for the first time but ultimately I want it to be in portuguese from the start so when the person gets the laptop they can just get started and not need to translate.

Neither am I, but that configuration is up to the buyer. Please follow my earlier instructions, What to do before you sell, give away, or trade in your Mac .... You will stop at the following passage in Step 6:

"If you want to leave the Mac in an out-of-box state, don't continue with the setup of your system. Instead, press Command-Q to shut down the Mac. When the new owner turns on the Mac, setup assistant will guide them through the setup process."

Press ⌘ Q and shut down the Mac as it states. If you do not follow those instructions exactly as written, its new owner will not be receiving a properly configured Mac. That was how it was delivered to you, and is exactly what the buyer should expect.

You should open Terminal app and type:

sudo languagesetup

Type your password.

You will get a numbered list of all the available languages.

Type the number for the Portugees language (is 12 in my list)

Push enter

Close the terminal app

Then restart your system.

Now the login screen should be in Portugees.

My guess is, that if you want a Portuguese version of Mojave, you will have to download it from the Portuguese Apple Store, make an installable USB drive and go from there.