Hi,
I haven’t built in support for handling the spellcheck in DFFS, but you should be able to trigger it in custom code by calling this built in function:
SP.UI.Spellcheck.SpellChecker.Spellcheck();
Maybe you can add a button / banner above the save button to tell the user to spellcheck before submitting?
Please note that I have only tested this in SharePoint online / Office 365.
Alexander