Thanks for the screenshot. Your setting is already correct: Portuguese is shown as the preferred Office display language, and Windows is also using Portuguese. So repeatedly changing “Set as Preferred” is unlikely to fix this.
This usually means the Portuguese Office display files are missing or damaged.
Please close all Office apps, then reinstall the exact Portuguese Language Accessory Pack you want (Portuguese Brazil or Portuguese Portugal). Make sure you choose the same 32-bit/64-bit version as your Office installation:
https://support.microsoft.com/pt-br/office/languages/language-accessory-pack-for-microsoft-365
After installing it, restart Windows, open Word > File > Options > Language, set Portuguese as Preferred again, then restart Word. Microsoft confirms that both Portuguese (Brazil) and Portuguese (Portugal) include the full Office display language.
If Word still remains in English, I would next run an Office Online Repair, as this can restore damaged language/display components:
Settings > Apps > Installed apps > Microsoft 365 > three dots > Modify > Online Repair.
https://support.microsoft.com/pt-br/office/lifecycle/repair-an-office-application
One detail I noticed in your screenshot: the authoring language is Portuguese (Brazil). If Brazilian Portuguese is the UI you want, please make sure you install the Portuguese (Brazil) accessory pack specifically, rather than Portuguese (Portugal).