Language does not switch when using fcitx mozc input method on Linux

Problem

With two languages enabled, the input language is always Japanese in any invoked popup (e.g. Search Everywhere), even if the input was switched in the editor to English before the popup opening.

Cause

This might be caused by faulty IM configuration.

Resolution

Please carefully check the following:

1. Correct variables and IM process are configured:

• environment variables in set | grep -E 'XMODIFIERS|GTK_IM_MODULE|QT_IM_MODULE';setxkbmap -query output are set to fcitx (see Fcitx input method doesn't work in JetBrains IDEs on Linux for more details on how to configure them);
• only the fcitx process is running when you check with ps auxww | grep -E 'ibus|fcitx' command in the OS terminal.

2. Both input methods - English and Mozc - are added to the list in fcitx settings (not only in OS Keyboard settings):
3. On the Global config tab in fcitx settings, the Share state among window set to All: