LanguageTool is an open-source proofreading tool for English, Italian,
French, German, Spanish, Portuguese, Dutch, and many other languages.
It detects grammar, style, and spelling errors that a simple spell
checker cannot find, such as subject-verb agreement problems, incorrect
articles, and commonly confused words.

Both command-line and GUI checkers are included, as well as an embedded
HTTP server to expose the LanguageTool API locally.  The server allows
clients such as Emacs (via flymake-languagetool or langtool.el), Neovim,
and browser extensions to request proofreading without sending text to
third-party services.