Sorry for the very late answer. I just saw this question today.

The correct answer to this would be to do it like described in this article (link), except it does not work on Sierra. That is, the installation works but Hunspell not. Your best option would be to downgrade the system to 10.11.+ (El Capitan) and install the directory after that.

If you however got this to work on Sierra, please report about it because more and more people are starting to ask for the same as they upgrade to Sierra.

https://creativepro.com/adding-hunspell-dictionaries-indesign.php