iOS keyboard language does not change in particular language

Question

I can change keyboard language from ja-JP to vi-VN by reference to iPhone: Change Keyboard language programmatically. But I can not change vi-VN to en-US on same code.

Changing Keyboard

I don't know why this happens but I fixed it. The fixing way is that I always set language ja-JP before changing other language(please refer to my project).

This project reproduce the problem. But enabling BEGIN: to END: block in func selectLanguage will solve the problem.

import UIKit

class ViewController: UIViewController {

@IBOutlet weak var textField: UITextField!
var lang = "en-US"

@IBAction func selectLanguage(_ sender: UISegmentedControl) {
    // BEGIN: Changing Keyboard language en-US to vi-VN is working if this comment this block out.
//        lang = "ja-JP"
//        self.textField.resignFirstResponder()
//        self.textField.becomeFirstResponder()
    // END:   Changing Keyboard language en-US to vi-VN is working if this comment this block out.

    switch sender.selectedSegmentIndex {
    case 0: // English
        lang = "en-US"
    case 1: // Vietnamese
        lang = "vi-VN"
    case 2: // Japanese
        lang = "ja-JP"
    default:
        break
    }

    // ReDisplay Keyboard
    self.textField.resignFirstResponder()
    self.textField.becomeFirstResponder()

}

override var textInputMode: UITextInputMode?{
    var tim = super.textInputMode

    for mode in UITextInputMode.activeInputModes {
        if mode.primaryLanguage!.localizedStandardContains(lang) {
            tim = mode
        }
        print("Available Keyboard: ", mode.primaryLanguage ?? "nil")
    }

    print("Selected language: ", tim?.primaryLanguage ?? "unknown")

    return tim
}

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.

}


}

I want to know correct way to fix this problem. Thank you.