LanguageTool

LanguageTool checks grammar, spelling, punctuation and style, and its main difference from most checkers is language coverage. It handles English plus German, French, Spanish, Portuguese, Dutch and many other languages, which suits people who write in more than one.

It runs as a browser extension, in a web editor, inside Google Docs and as an add-in for Microsoft Word, with desktop apps as well. There is a free version, paid plans with more checks, and a developer API for adding checks to your own software. Its core engine is open source.

Rules differ in depth from one language to another, so it may catch more in some languages than in others. Test it on a few of your own documents in each language you use before you rely on it.