LanguageTool

LanguageTool is a German open source writing assistant founded in 2003, originally developed as an academic project by Daniel Naber at the University of Bielefeld. Now operated by LanguageTool GmbH based in Potsdam, Germany, the tool has evolved from a simple grammar checker into a sophisticated AI-powered writing assistant that supports over 30 languages -- far more than any US-based competitor. Its open source heritage and multilingual focus make it uniquely suited for the diverse linguistic landscape of Europe.

The platform offers comprehensive grammar checking, style suggestions, tone detection, and AI-powered paraphrasing capabilities across its supported languages. LanguageTool integrates seamlessly into daily workflows through browser extensions for Chrome, Firefox, Safari, and Edge, as well as add-ins for Microsoft Office and Google Docs. It also provides a standalone editor, an API for developers, and the ability to create custom dictionaries. The open source core (available on GitHub under LGPL) can be self-hosted by organizations requiring full control over their data and infrastructure.

Headquartered in Potsdam, Germany, LanguageTool processes all text on German servers and is fully GDPR compliant. The company offers a free tier for basic checking and a premium plan that unlocks advanced features including longer text support, style improvements, and team functionality. For European users seeking a privacy-respecting, multilingual alternative to Grammarly that truly understands European languages beyond English, LanguageTool offers a compelling combination of open source transparency, genuine multilingual competence, and German data sovereignty.