There is an alternative solution (Wproofreader) for check spelling and grammar in ProseMirror that you can consider: https://webspellchecker.com/wsc-proofreader/#proofreader-prosemirror
I have just published a post about it: WProofreader Spelling and Grammar Checker for ProseMirror
This is not an open source solution but it provides nice benefits along with the extensive language support.