Affinity Publisher uses the Hunspell Engine for spell checking and it also allows for additional Hunspell dictionaries to be added. You can download additional dictionaries from LibreOffice's GitHub page here.  When you find the file that you want to download you will need to view the file on GitHub. Depending on how big the file is you will see either a RAW or Download button. If you see the RAW button you will need to right click the RAW button and select the option to Save Link As... to down