Based on Linting Prose in Emacs…
Needs proselint installed:
pip install proselint
Also needs a flycheck checker defined:
(flycheck-define-checker proselint
"A linter for prose."
:command ("proselint" source-inplace)
:error-patterns
((warning line-start (file-name) ":" line ":" column ": "
(id (one-or-more (not (any " "))))
(message) line-end))
:modes (gfm-mode
markdown-mode
org-mode
text-mode))
(add-to-list 'flycheck-checkers 'proselint)