There is an easier solution, and you are still able to use the pt-BR lang just fine. I went online and found the solution, gonna keep it for posterity here if anyone needs it.
This is caused by an error in the preferences.json file, where it should be pt-BR it is pt_BR which causes the error to fix this is quite simple:
1. Open the preferences.json file which is located C:\Users\{YourUser}\AppData\Roaming\itch for windows users (You can also access this by using Windows + R and type %appdata% , or you can just put that in the big bar at the top of your explorer) and ~/Library/Application Support/itch for MacOS users.
2. locate and change the pt_BR for pt-BR and you should be good to go after quiting and closing the app.
I honestly hope that the devs can correct this thing, it shouldn't take long for it, a simple update would do. Anyways, hope it helps!