Hello everybody.
Since LibreOffice 3.5.0 there is a New grammar checker for English. It would be great if a grammar checker for German (and other languages) comes with the next major release.
The DocumentFoundation wiki has a page with tools organized by language:
http://wiki.documentfoundation.org/Language_support_of_LibreOffice
Based on the information on that page, LanguageTool might be what you’re looking for:
http://extensions.libreoffice.org/extension-center/languagetool
“LanguageTool is a style and grammar proofreading software for English, French, German, Polish, Dutch, Romanian, and a lot of other languages. You can think of LanguageTool as a software to detect errors that a simple spell checker cannot detect, e.g. mixing up there/their, no/now etc. It can also detect some grammar mistakes. It does not include spell checking.”
v1.7 from March of this year states that it is compatible with LO 3.5