Where can I get a Dutch language pack for spelling correction for the Windows version of Publisher?
Anything built for Hunspell should work, which means anything built for OpenOffice or LibreOffice, too. The LibreOffice collection on GitHub is often a good starting point, and you can find their Dutch set here: https://github.com/LibreOffice/dictionaries/tree/master/nl_NL
You need the .aff file and the two .dic files.
Edit: Downloading the files is tricky. If you're not careful you end up downloading an HTML page instead, which won't work. This approach should work:
Click on the file you want (say, hyph_nl_NL.dic). That will display the file contents, or tell you it's too large.
If it displays the contents, right-click on the Raw button, and choose Save File.
If it tells you it's too large, right-click on the Download button, and choose Save File.
After downloading them, open Publisher's Preferences/Tools, and near the bottom of the panel on the Additional Dictionaries setting, click Open:
FIle Explorer will open. Create a new directory nl_NL, and place the 3 files downloaded from GitHub in it. Then all you should need to do is restart Publisher, and you will be able to set Dutch as a spelling and hyphenation language using the Character panel or when defining Text Styles.