Does the app have a spell checker? Or, can an app like Grammarly be used with it?
Diarium uses the default system spell check that is provided by the operating system
The default is 'so, so.' On to my second question. Is Grammarly compatible with Diarium?
Unless Grammarly can plug in via their app (do they have an app?) into other apps, I don't think so - I never tried out that service. I didn't add any integration into Diarium either.
As a Grammarly user, I can confirm the Diarium uses the default and works like a charm