I’ve mapped Caps Lock to EN and Shift + Caps Lock to RU. Really stopped thinking about “what keyboard layout is on rn”, just subconsciously press Caps every time I start typing something in English. Pretty easy to replicate with Karabiner on macOS (and on Linux it’s supported out of the box).

If you really want a dedicated keyboard though yeah, would be a bit harder but might be possible to do something with Karabiner as well. The device_if condition might work here: https://karabiner-elements.pqrs.org/docs/json/complex-modifi...

Edit: this might work but I haven’t tried:

    {
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "any": "key_code",
            "modifiers": { "optional": ["any"] }
          },
          "to": [
            { "select_input_source": { "language": "^ja$" } },
            { "from_event": true }
          ],
          "conditions": [
            {
              "type": "device_if",
              "identifiers": [{ "vendor_id": 1111, "product_id": 2222 }]
            }
          ]
        }
      ]
    }