I ran into an unusual Mac OS bug this morning. I hit the kana key to type an address in Japanese, but it came out in romaji. I hit it again — same thing. When I looked in the input menu, I saw that all the Japanese and Korean input options had disappeared. I attempted to add them again in system preferences but they didn’t appear as choices. The Chinese options were also missing. Oddly, when I logged into another user account on the same machine, everything was where it should be. Multiple restarts did nothing, nor did changing my main language to Japanese.
I finally found a mention of something similar relating to High Sierra in a blog post that recommended doing a safe boot to set things right.
I tried it and it worked. Phew! I have no idea what triggered it, though.