We have created a multi-language spelling and grammar checking solution for ProseMirror. You can check the demo here: https://webspellchecker.com/wsc-proofreader/#proofreader-prosemirror
WProofreader allows end users correcting mistakes as they type inside ProseMirror, or hasteless proofreading of all the text at once in a separate dialog.
Besides, users can create their personal user dictionaries with rare or field-specific words. Such words will be remembered and not reported as mistakes and available as corrections.
WProofreader provides support for all modern browsers and can be used on touch devices.
You can subscribe for a trial license key here: https://webspellchecker.com/wsc-proofreader/
I’m open to hear you questions and feedback.