Ubuntu – Korean (hangul) text entry not working Ubuntu 18.04

Assigning a key to another is unique when the key is a modifier. This answer has a good explanation:

The action, when it is a modifier action such as this, changes the current modifier state, which (simply put) is a set of flags recorded in the operating system that record what modifiers are currently "on". [This record] influences what action further keypresses map to.

And so when you are trying to map your key, it is being mapped as a regular key and losing information which specifies how it behaves when multiple keys are pressed.

To demonstrate, run xmodmap. Note that there are two default keys (Control_L and Control_R) mapped to the control modifier. If you run xmodmap -e 'keycode 131 = Control_L' and then xmodmap, the control modifier should still have two keys, so your key isn't being recognized as a modifier.

To map as a modifier:

Clear the modifier key control

xmodmap -e 'clear control'

Add the keycode for the Hanja key

xmodmap -e 'keycode 131 = Control_L'

Lastly, add back the default modifiers:

xmodmap -e 'add control = Control_L'
xmodmap -e 'add control = Control_R'

Now, if you were to run xmodmap again, you will notice a new key has been added for the modifier control, and it should work correctly now.

Here is a link to official Arch Linux instructions on reassigning modifiers to keys on your keyboard.

edit:

for persistent changes, add to /usr/share/X11/xkb/symbols/pl:

key <HJCV> { [ Control_L ] };

modifier_map Control { Control_L, Control_R, <HJCV> };