Karabiner

Fast change language without switch popup.

Install Karabiner-Elements. Create Complex Modification rule:

{
    "manipulators": [
        {
            "description": "fn -> previous input source.",
            "from": { "key_code": "fn" },
            "to": [
                {
                    "key_code": "spacebar",
                    "modifiers": ["left_control"]
                }
            ],
            "type": "basic"
        }
    ]
}

Got ot Parameters tab and set value of to_if_alone_timeout_milliseconds to 0.

Final config file will be:

karabiner.json

{
    "profiles": [
        {
            "complex_modifications": {
                "parameters": { "basic.to_if_alone_timeout_milliseconds": 0 },
                "rules": [
                    {
                        "manipulators": [
                            {
                                "description": "fn -> previous input source.",
                                "from": { "key_code": "fn" },
                                "to": [
                                    {
                                        "key_code": "spacebar",
                                        "modifiers": ["left_control"]
                                    }
                                ],
                                "type": "basic"
                            }
                        ]
                    }
                ]
            },
            "name": "Default profile",
            "selected": true,
            "virtual_hid_keyboard": {
                "country_code": 0,
                "keyboard_type_v2": "iso"
            }
        }
    ]
}