Thanks guys. Claude is correct, I am using his LOS 16 ROM. He has done a great job maintaining it!
This has been very frustrating, but I have finally solved the problem. For reference, I removed Swiftkey and reselected the AOSP keyboard and it roared back to life. Have no idea why because, as far as I can tell, while Swiftkey has word prediction, it uses Android's dictionary (system and user). I then reinstalled Swiftkey and it continued working. Again, I don't know what caused it, but this fixed it.
Just like in Claude's screenshot, mine now says "Use system languages" under Languag