Mapping keys to set different input languages on macOS

2021-08-15

Prerequisite

You need Karabiner installed.

Final Goal

Press Left Command to set Mac's input method to English.
Press Right Command to set Mac's input method to Korean.
Any other shortcut combinations would perform as usual.

Instructions

Go to ~/.config/karabiner/assets/complex_modifications. You can click Command+Shift+G within Finder to open a goto window.
Create a JSON file like the following here (open any text editor and save it as filename.json).
Go to Karabiner-Elements.app → Complex Modifications and press add rules.
Click the rules that you want to enable. The above text file will show under Multilingual Input Methods.

json

{  "title": "Multilingual Input Methods",  "rules": [    {      "description": "R Command to 한글",      "manipulators": [        {          "type": "basic",          "from": {            "key_code": "right_command",            "modifiers": { "optional": ["any"] }          },          "to": [{ "key_code": "right_command", "lazy": true }],          "to_if_alone": [            {              "select_input_source": { "language": "^ko$" }            }          ]        }      ]    },    {      "description": "L Command to English",      "manipulators": [        {          "type": "basic",          "from": {            "key_code": "left_command",            "modifiers": { "optional": ["any"] }          },          "to": [{ "key_code": "left_command", "lazy": true }],          "to_if_alone": [            {              "select_input_source": { "language": "^en$" }            }          ]        }      ]    }  ]}

{  "title": "Multilingual Input Methods",  "rules": [    {      "description": "R Command to 한글",      "manipulators": [        {          "type": "basic",          "from": {            "key_code": "right_command",            "modifiers": { "optional": ["any"] }          },          "to": [{ "key_code": "right_command", "lazy": true }],          "to_if_alone": [            {              "select_input_source": { "language": "^ko$" }            }          ]        }      ]    },    {      "description": "L Command to English",      "manipulators": [        {          "type": "basic",          "from": {            "key_code": "left_command",            "modifiers": { "optional": ["any"] }          },          "to": [{ "key_code": "left_command", "lazy": true }],          "to_if_alone": [            {              "select_input_source": { "language": "^en$" }            }          ]        }      ]    }  ]}

Configuring more languages

Add a JSON Property with your language of choice. The language identifier is likely some combination of ISO 639 and ISO 3166 (i.e., BCP 47).
https://karabiner-elements.pqrs.org/docs/json/complex-modifications-manipulator-definition/to/select-input-source/

Update Mar 7th, 2022

If you're bilingual, try looking into Eikana.
I switched to Eikana + Gureum configuration; there's a notorious 자소 분리 bug in macOS Korean Keyboard.