This happens when setting the language to Chinese in a newer version of Spine, then going back to an older version of Spine. If you restart Spine the error may go away. If not, you can delete your prefs.json file, then start Spine:
Windows: <user home folder>\Spine\prefs.json
Mac: <user home folder>/Library/Application Support/Spine/prefs.json
Linux: <user home folder>/.spine/prefs.json
It is a bug with older versions not being able to read the newer prefs.json file. Since we don't do updates to older versions, unfortunately we aren't able to fix it. However, we've improved how this works in v3.9+.
Thanks for reporting!