If you prefer not to use the Galaxy client, you can download the standalone installer for the language pack. Log in to your account on the GOG website Go to your The Witcher 3: Wild Hunt to expand the download options. Look for the section labeled Language Pack Installers Portuguese (Brazil) in the list and download the installer file.
While the keyword suggests a direct download, you should be cautious.
(Optional) Change and Text Language to Portuguese as well. Troubleshooting Common Issues
