LanguageTool

It provides open‑source proofreading for more than 20 languages, detecting grammar, punctuation, spelling, and style errors that simple spell checkers miss. The core software can be run locally as a standalone application, a command‑line tool, or an HTTP server, and it offers language APIs for Java and other environments.

The project is released under LGPL 2.1, is self‑hostable, and does not require a subscription. Docker images and an installation script are provided to simplify deployment, and the repository includes documentation for building from source with Java 17 and Maven.

Typical users are writers, editors, and developers who need multilingual error detection integrated into text editors, office suites, or custom applications, while retaining control over data privacy by hosting the service themselves.