Over the last years Neovim became my primary editor for almost everything. I edit a lot of markdown files, so I obviously need a spell-checker. I already new about including LanguageTool through LSP, but found it too much effort, since it relies on a local installation and I am already running LanguageTool in my local network. So, to no surprise I just learned that Neovim already comes with a basic spell-checker, which is good enough for my purpose.
You can enable it with :setlocal spell and disable it again with :setlocal nospell - awesome.