I'm very concerned about privacy with this extension. As an example of what they collect (from their privacy policy).
Usage information (such as the date and time of visits, pages viewed, links to/from any page, and time spent in a session)
Device information (such as your internet protocol (IP) address, Internet service provider, operating system, computer model, language settings and CPU details, referrer URL, time zone information, data volume transferred, access status/ HTTP status code, and web browser type).
Inference Information about your usage of LanguageTool (such as whether you are a frequent document uploader or user of a particular feature). Location information (approximated, based on your IP address).

Almost none of this has anything to do with helping to check spelling and grammar and it goes way beyond what anyone should be willing to share to any company, let alone an extension.