~/.config/karabiner/assets/complex_modifications

custom_layout_switch.json

{
    "title": "Custom Layout Switch",
    "rules": [
      {
        "description": "CapsLock to switch to English layout",
        "manipulators": [
          {
            "type": "basic",
            "from": {
              "key_code": "caps_lock"
            },
            "to": [
              {
                "select_input_source": {
                  "input_source_id": "org.sil.ukelele.keyboardlayout.t.english-ilyabirmantypography"
                }
              }
            ]
          }
        ]
      },
      {
        "description": "Shift+CapsLock to switch to Russian layout",
        "manipulators": [
          {
            "type": "basic",
            "from": {
              "key_code": "caps_lock",
              "modifiers": {
                "mandatory": ["shift"]
              }
            },
            "to": [
              {
                "select_input_source": {
                  "input_source_id": "org.sil.ukelele.keyboardlayout.t.russian-ilyabirmantypography"
                }
              }
            ]
          }
        ]
      }
    ]
  }