After (automatically) installing 2019-09 Cumulative Update for Windows 10 Version 1903 (KB4515384), the language bar disappeared after login. On the login screen it shows normally and can be used to switch languages. Running ctfmon has no effect. In addition, in the Language options, when choosing 'Add a keyboard' for Chinese (Simplified, China), all of the two keyboards are gray and cannot be added. Another new language (tested Chinese Traditional) can be added but when adding a new keyboard all options of keyboards are also gray.
After uninstalling KB4515384 in appwiz.cpl (Programs and Features control panel) and restarting, the language bar appears normally and adding a new keyboard option is no longer gray.
Windows edition: Enterprise
Windows version: 1903 Build 18362.295 (after uninstalling the update)