I keep using LibreOffice for just the spell & grammar check, earlier it was MS Word.
Is there an offline version for English language that is multi-platform?
Basically an app like Windows Notepad with spell & grammar check.
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Try the CudaText editor (cross-platform, open source).
It has plugin Spell Checker, it is Hunspell based. Plugin shows menu items in Plugins menu.
Notes on the Mac has spelling and grammar checking. Most Mac apps have this. Goto the edit menu and there is a spelling and Grammar selection. You can turn these on by selecting options you want. I don't know how well these work. Since this seems to be a system wide option it works with Textedit too.
You can also get a free account with Grammarly. It runs in background but monitors everything you type.