I got the LibreOffice plugin from here, but it’s working out-of-the-box and I don’t understand how.
Where is the server?
Is it using LanguageTool remote cloud server?
If so, shouldn’t this be clearly specified in the webpage, like it’s specified for the Thunderbird plugin.
Local server: You can set up a local server as described at LanguageTool embedded HTTP Server | dev.languagetool.org (requires Java). Once it’s running, visit the add-on settings and point it to localhost under “Experimental settings”.