@ Patrick Conner
Do you mean the software user interface in Dutch or spell checking of text you create?
Mojave is the latest version of macOS. I think the reference to iOS was a slip of the ‘pen’.
Yes, I meant the Operating System Mojave for the iMac desktop computer
Selecting the program, showing Package content and adding nl-NL dictionary to Affinity Photo | Contents | Resources | Dictionaries did not mean Dutch appeared in Affinity Photo Preferences | General | Language.
Fooling the program by changing the contents of for instance hyph_de_DE.dic by hyph_nl_NL.dic also did not work.
The dutch dictionary files were the same (Windows only?) files as Walt Farrell put in the link to a ZIP file in his post of 7 July