Ctrl + , (for Windows)
File / Options
. . . . . . .
Hmm, I just opened TextEdit and no red squiggles there! I went to Edit > Check spelling while typing, no red lines and when I checked the menu again check spelling wasn’t selected.
I just checked in TextEdit and the check spelling is not working there either 
Interesting. So either Apple broke something in a recent update (I’m on 13.4 and it still works as expected) or… good question. You already checked the spell checking setting under system settings → keyboard → input methods, right? (“automatic” should be fine). Do you have any third-party tools that would try to “hijack” an editor? Something like grammar checkers.
It’s very odd, right?
And yep, I checked input methods… everything looks good there. No grammar checkers, either.
You could give this (terminal) command a try:
defaults write -g NSAllowContinuousSpellChecking -bool true
(I’m not sure if this works immediately, might be nessesary to reboot.)
EDIT: Since the OP can’t reply right now (posting limit apparently), the last idea fixed it. For whatever reason.
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This worked for me! I happened to Google how to fix spell check issue on Mac if you used a third party tool just for the heck of it (figured whatever reset would work in that case could help me) and I found this: Michael Tsai - Blog - Grammarly Disables Spell Checking Globally
And it worked! The link @November_Sierra shared is the same link the author of the blog post shared. I don’t have any third party spelling/grammar tools on my Mac but it still worked for me, hopefully it’ll work for anyone who stumbles across this thread!
Thank you so much for leading me in the right direction @November_Sierra
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I had the same problem and typing this command into terminal on my mac worked for me too
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