languagetool-org/languagetool is an open-source correction engine that checks style, grammar, and spelling errors in more than 25 languages. It is written in Java and released under the LGPL-2.1 license. However, no release version is given in this material.
The README explains that it supports English, Spanish, French, German, Portuguese, Polish, Dutch, and more than 20 other languages. Catching errors that a simple spell checker alone cannot find is the aim of this project. It finds writing errors with per-language grammar rules and an API, and the HTTP API is the way for external programs to send the text they want checked. The README covers both how to run your own server and how to use it from Java. The repository description presents this project as a style and grammar checker. The repository was created on August 8, 2013 and has 15165 stars.
That said, the evaluation score is low, with momentum at only 3.0 and no values given for the novelty, technology, and quality items. Because the material contains no performance measurements or comparison targets, it is hard to judge which errors it catches and how many. There are no records of false positives or misses either. Only the LGPL-2.1 terms and the 25+ language coverage are confirmed, so before adoption it is worth looking separately at the detection range for each supported language.