Install aspell-pt-br by entering the following commands in the terminal:
sudo apt update sudo apt install aspell-pt-br
Description:
Brazilian Portuguese dictionary for GNU Aspell
This package contains all the required files to add support for the Brazilian Portuguese language to the GNU Aspell spell checker.
Homepage: http://pt-br.libreoffice.org/projetos/projeto-vero-verificador-ortografico
Version: 20131030-10
Section: universe/text