It DOES still need to send data somewhere to check
Your privacy is important to us: By default, this extension will check your text by sending it to languagetool.org over a securely encrypted connection. No account is needed to use this extension. We don’t store your IP address. See languagetool.org/privacy/ for our privacy policy.
I don’t want to confirm details I don’t know, so someone else should probably explain more on if this is good/bad