Grammarly has a terrible privacy policy, so you are right to be cautious. Unfortunately I don't have any good alternatives to offer as I only use spellcheck myself.
This one is recommended by Mozilla
https://addons.mozilla.org/en-CA/firefox/addon/languagetool/
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 https://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
It's on my to-do list, but you can also spin up your own language tool instance so that your data never leaves your house, since it is open source: https://github.com/languagetool-org/languagetool
If you have a homeserver it can go there, otherwise you can also run it on your computer although I am not sure how much RAM it will use.
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You can run LanguageTool locally. While it isn't as great as the paid version, I use this to check nearly everything I write for work in my native language, and in the other languages I speak
https://caderek.github.io/gramma/ is a cli spellchecker that has the option of installing a LT server locally. Not ideal if you are writing things with Pages/Word/etc., but a possible backup.
You can run your own instance of this?
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Very true.
Privacy has become a very important issue in modern society, with companies and governments constantly abusing their power, more and more people are waking up to the importance of digital privacy.
In this community everyone is welcome to post links and discuss topics related to privacy.
[Matrix/Element]Dead
much thanks to @gary_host_laptop for the logo design :)