LanguageTool is a free open source proof-reading software for English, French, German, Italian, Japanese and more than 20 other languages. It finds many errors that a simple spelling checker can not detect as well as grammar problems and is used in some CAT tools such as OmegaT.
If your language is not supported, you can read the three-minute introduction for writing error detection rules. Of course, you can also improve the rules for existing languages and add new rules. With the LanguageTool Rule Editor (an online tool) you can create your rules if you don’t want to edit the XML file directly. Below is also a short introductory video about the creation of rules.
LanguageTool is available as Standalone Software for your desktop and as Add-on for Firefox, Chrome, Google Docs, LibreOffice/OpenOffice.