Just wondering if there is a way to enable spell checking in Xcode. Maybe an add-on or something...
Spell checking was broken in Xcode 9 and 10. It's now fixed in Xcode 11.0.
(Edit ► Format ► Spelling and Grammar ► Check Document Now)
Original answer below:
Unfortunately it doesn't work in Xcode 9 and 10. I posted a bug to Apple using "Bug Report". And it was marked as a dupe of a bug with number "32062963". And I found it here.
So if you want it works, PLEASE, POST A BUG! here.
Apple uses this to gauge how serious a bug is.
As it was mentioned here by Donarb.