I'm happy to announce LanguageTool Plus, a premium version of LanguageTool. For now, its focus is on German, for which it offers improved error detection. Check out the pricing table at https://languagetoolplus.com to see what exactly it can do today - it's constantly being extended. LanguageTool Plus is not available for download, it's a hosted service. Its API is compatible to LanguageTool and you will soon be able to use the existing browser add-ons with LanguageTool Plus.
Consider buying a LT Plus subscription, you'll get premium features and it also helps to move LT forward. For example, LT Plus currently helps to improve the browser add-on, which will have a new release soon. LanguageTool (languagetool.org) will of course continue to be developed, hopefully even faster than before.