DavidXanatos Microsoft Word, Google Doc, LibreOffice Writer, etc., all have spell checkers. Another option might be writing in German and using a translator app. An ongoing problem with general-purpose LLMs is that, unless one is extremely careful about checking the output, plausible-seeming junk will creep in. In May, Anthropic's lawyers were in hot water because one of them asked Claude (Anthropic's LLM!) to properly format a legal citation to a real scientific paper, and instead it provided a citation for a paper that never existed (source).
If one has the knowledge and time and patience to carefully check every word that comes out of an LLM, they can be useful, but at present the key thing to know is that they are fundamentally unreliable.