When we try to manually translate this page to Portuguese, all the text fields are not available for translation as they are when we try to translate to other languages.
Please confirm that I understood your issue correctly.
When you use manual translation, the original texts are not there for you to translate them?
If what you want is for the default language texts to show in the Portugese page so you can translate them, you can do that by clicking the "Copy content from English" button in the "Language" section of the page, when you are editing the Portugese version. That will copy over all the content, and then you can change it as you want.
I have attached a screenshot showing this, with Spanish being a secondary language.
If I misunderstood you or this does not solve your issue, please do not hesitate to reply.
If so, please try to explain your issue in as much detail as possible, attaching screenshots where possible.