Running LanguageTool locally is a bit of a pain, with some manual steps. Plus you have to fetch some data files. You can find around a few projects like this one to make it easier to run LanguageTool.

And yes, as the poster mentioned, LanguageTool keeps some code exclusive to their paid version. There's a bit of a tension because they ask people not to extend OSS LanguageTool with their paid features.

There's also this interesting clone, but it seems abandoned.