If you wish to set-up aspell on a per-project basis, several variables have to be configured accordingly.

The following procedure considers that your spellchecking engine of choice is aspell, and you’re using spell-fu to show the typos in your buffer.

The first step is to make sure you have aspell correctly installed in your system. Type aspell -a -m to check if it was installed correctly. If not, you can do it on Arch Linux like this:

pacman -S aspell aspell-pt

The above command installs aspell along with its Portuguese dictionary.

The second step is make sure you have added the (spell +aspell) line on your Doom Emacs init.el file. After that, create a file named .dir-locals.el in your project root, and add the following lines:

;;; Directory Local Variables            -*- no-byte-compile: t -*-
;;; For more information see (info "(emacs) Directory Variables")

((org-mode . (eval . (spell-fu-dictionary-add (spell-fu-get-ispell-dictionary "pt_BR")))))

Here, the dictionary language is set to Brazilian Portuguese.

After these steps, you should start seeing misspelled words underlined in red. To navigate through them, press ] s and [ s to move to the previous and next words, and then z = to correct them.