Is there any way to change language input icons on macOS?

Some languages like Spanish and Russian use the country flags for language input, while others like Chinese and Japanese are not. So I would like to change language input icons such that all languages have the country flags for aesthetics. I also want to set different icons to different input system on Japanese.

Is this possible on macOS?

(I use Big Sur Beta 2 but a solution on Catalina is totally fine.)

The only way to do this as far as I know is to create custom keyboards with a app like Ukelele, where you can create any icon you want for a layout. But Ukelele cannot make custom keyboards for complex "input methods" like Chinese and Japanese.