After answers form users about fixing this "feature" by terminal: sounds good, but difficult.
OPTION 3:
1) Open activity monitor window (command + space: activity monitor.app)
2) Find process: TextInputSwitcher (in memory tab(it's second tab. First tab: CPU); or tap in search field: TextInputSwitcher)
3) Stop process TextInputSwitcher (Click the Stop button
in the upper-left corner of the Activity Monitor window: Force stop)
4) Close activity monitor (command + Q)
After: Tap by mouse cursor on language menu (top right corner), and change languages few times. After "Fn" button will start work again automatically.
P.S. Sorry, I haven't permissions to fix it with a macOS update. I'm not Steve Wozniak. Good luck.