I suggest that the text is merged with the headline, thus:
LanguageTool is proofreading software for more than 20 languages. It finds grammar errors, spelling errors, and bad style.
'Service' implies software-as-a-service (SaaS), but LT is available as a download.
A 'simple spell checker' cannot find the errors that LT finds. Can an 'advanced spell checker' find the problems? Why mention spell checkers?
I think that the input box should be smaller, so users immediately see that downloads are available.