When you use several languages on your computer, there is a curious problem in OSX 10.13.6 High Sierra, that I haven’t experienced before. With Japanese as one of the languages selected as ‘input sources’, my Mac begins to behave in an unstable way.
The symptoms include
very slow or no response to keyboard commands;
unexpected delay in response
and repeated keyboard strokes when non were executed by myself.
Sometimes the Mac stalls completely with or without the ‘wheel of death’.
It is so bizarre that at first I thought I was being hacked or some virus took over my machine.
After some discussions with fellow macusers and research I found that it was not just my machine. Many others have this problem as well.
Apparently this issue occurs when a language which uses many accents, like French in my case, clashes with Japanese kotoeri input system (you type with Latin characters and the computer changes them into Japanese characters).
So, the solution
is to remove either Japanese, or the other language from input sources.
To do that, go to System Preferences > Language & Region and click/tap on Japanese/Nihongo.
Then, click on the minus symbol below the languages side-panel.
Then, restart the computer and it will come back to its usual stable behaviour.
If you need Japanese rather than French, try removing French. I use Japanese only occasionally, so I can’t vouch for this. But removing Japanese has worked for me.
Meanwhile, we should wait for the Apple wiz-people to sort the problem. Ahh, well.
See this article for a slightly different description of the same problem and similar solution.