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(s) that you want to download please make sure you view the file on GitHub and click the RAW button. This will make sure you're not viewing the HTML version of the file and download the correct file. Now that you're viewing the RAW data of the file, you can right