Is this extension send text to ge languagetool.org servers for checking?

Great! If you don't like sending your data to a third party network you can use this add on with a locally hosted instance on your computer or on a private network.

I simply run language tool from the command line like this:

languagetool --http --allow-origin "*"
default port is 8081 but you can specify another one by adding this : --port 666

then in the add on preferences simply change the LanguageTool API Server URL to:
http://localhost:8081/v2/

LanguageTool is a very useful spell- and grammar checker. It is easy to use and open source. A must have if you want to write in a language that is not 100% in your grasp.

The new version significantly improved the usability of this very useful add-on! Keep up the good work!