LanguageTool is a free grammar checker and paraphraser for English, Spanish, and 20 other languages. Instantly check your text for grammar and style mistakes.
Note about deepl: according to their EULA they collect all what you type there and use it for training their algorithm. They are much smaller that Google and co, what makes me thinking that your data could be more "visible" in the training results.
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.