Hey, I wrote a plugin to run the languagetool spell- and grammarchecker  in Fidus Writer . Like our core code, it  is currently licensed AGPL, but if there is interest to collaborate on continued development of it from one of the other ProseMirror-based editors, we could split the prosemirror-related code out and license it under LGPL and then work on that part together with you!
Thanks a lot, let me study it
We’ve created a ProseMirror integration for ProWritingAid, our grammar checker and style improvement software. The integration with ProseMirror is open source so you can adapt it to other grammar checker APIs if you’d like. Or you can use it by signing up here: https://prowritingaid.com/en/App/BeyondGrammar
You can find the code here:
Nice! Too bad it’s written in TypeScript (?). Maybe we can make use of it some time in the future. Right now ours is working quite well and adding more languages/dialects to the mix would make it even more difficult to bring in new developers, so for the moment it seems like we would just spend more time and not gain much by switching to this.
Btw, I see now you mention license fees? How is that possible if it’s MIT licensed?
The source code is open source. You can do with it as you will. Even if that involves integrating another grammar checker. License fees apply if you want to use our grammar checker as you’d need to connect to our servers.
I was thinking that other people might want to use it rather than you as you already have one integrated into your product.