I had this issue in 5.1. The spell checker was removed from the Rich Text Editor. I had found a company call Web Spell Checker https://webspellchecker.com/ A license is required and therefor is fee based however.
Due to the nature of our business this company provided us with a custom build. Their response and support was fantastic.
We were able to install their 30 day trial with minimal effort although we did have to contact them to get it to "talk" to APEX, but like I said they were very responsive and we even did a screen sharing session to get it integrated with the Rich Text Editor.
This may be an option for you.
Thanks Joe R for your input.
What happen after the 30 day trial, I assume you had to upgrade to full license...
I just wonder why APEX Team does not try to integrate it, since it's very helpful...
anyway, will have to talk to our Manager first...
Yes. We bought the application and hosted it ourselves. If you looked over the link I provided you'll see they have 2 licensing packages.