[ Thanks to Nobody for this link.
]
“Although LanguageTool is probably not on par with the
grammar checker offered by Microsoft Office or other commercial
closed source office suites, it does have one important advantage:
you can easily define new grammar rules. The current version of
LanguageTools supports several languages besides English, including
German, Polish, French, and Dutch. The degree of support varies
from language to language; right now the most supported language is
Polish (it includes 579 grammar rules), followed by English (221
rules) and Dutch (198 rules).”