LanguageTool is a free grammar checker and paraphraser for English, Spanish, and 20 other languages. Instantly check your text for grammar and style mistakes.
Their extension isn't open source anymore, see here, so I don't recommend it personally, especially with how sensitive the data it collects is, its basically a keylogger, so trust is super important imo.
While I appreciate the sentiment, isn't that effectively the same as "if the restaurant won't give you the complete recipe for their pizza sauce, how do you know they're not trying to poison you?"
I think this is only the case with the free version. Source: I worked with some big German companies which wanted to have an autotranslate function for their software. DeepL was the top choice, because the quality is very good and the data protection agreements for the paid service left no questions.
They have total different use cases. DeepL is a translator, language tool checks for style and grammar.
I use both frequently and both do a very good job. I have a prime membership of language tool and I like it. Both are German companies and operate under GDPR.
It's working pretty well for me with integration into Firefox, (libre)office and you can decide if you want to run your own server locally, or rely on their server as a premium subscription.
First time I've heard someone say they're jealous of an America's free schooling system lol. Even in deep Africa villages they have free schooling systems, so idk where you are where that doesn't exist.