I don't use Avast One, but my guess would be very similar to Avast Free. First you would have to download the Portuguese language pack and select that as the default language.
That said According to an Avast FAQ Avast One is only available for download in certain countries.
https://support.avast.com/en-us/article/One-FAQ/Currently, Avast One is only available for download in the following countries:
Americas: Canada, The United States
Asia & Pacific: Australia, New Zealand
Europe: Austria, France, Germany, Ireland, Switzerland, The United Kingdom
So I don't know how you got it, but I'm guessing that Portuguese, probably isn't available as a supported language pack.