the firefox translations addon is also surprisingly decent for translations if you’re using firefox, works fully locally and the only external network connection being made is to download language sets. Can translate pages as well as custom text.
Previous LanguageTool worker there.
I’ve been contracted for a couple of months to help with the implementation of more French rules into the tool.
The only data we had, at least for me, as a rule editor, were mostly just expression and words we received to be incorrect, or new words added to user lexicons. Only the most common ones were available to us, as more unique ones are more likely to contain private info and be irrelevant to an actual issue with the tool or set of rules.
In addition, you can totally download the LanguageTool server to use it offline, including via the extension (and all other official plug-in implementations if I’m correct) to use your own self-hosted instance.