Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
MAC OS X Sierra (10.12) was released only 5 days ago or so, as of date of writing.
There is very little info on app compatibility available as yet!
Merriam-Webster Dictionary app is not yet rated as compatible on Roaringapps.com's listings.
The same lack of information for grammarly app: not even listed in Roaringapps.com. They list Merriam-Webster Dictionary, in fact, as not certain to be compatible with even OS X 10.9, so you might say they have no info.
I can find no information. And I suspect that, aside from trying it yourself, there is no way of finding out at this time, I'm sorry to say. 5 days is not a long time for everything to become posted on the internet, even for a widely used OS like MAC OS X, and even for a fairly standard app like Merriam-Webster Dictionary.
Does this answer your question? may I help any further with this matter?