Android Studio – Android Studio Flamingo not well configure

I’m starting to learn Android (I have some base in java) and I’m using the last version of android studio. Firstly there is no way to change language from kotlin to java (or it’s very well hide). Every video that I watched are on older versions of android studio and everything is different.
I decided to use kotlin but it still not working (video still don’t show the same things that I have). Here is some code that should show "Hello World" that I found on the Internet but there is a lot of error. So I suppose Android Studio is not correctly configure :

xml:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="example.app.javatpoint.helloworld.MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</android.support.constraint.ConstraintLayout>

Error:
android.support.constraint.ConstraintLayout (both)

kotlin:

package example.app.javatpoint.helloworld

import android.support.v7.app.AppCompatActivity
import android.os.Bundle

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
    }
}

Error:
v7 /
AppCompatActivity /
override /
onCreate /
setContentView /
R

Can you please also tell me which language is the best (I would like to create android game).

(Sorry for my bad English, I’m French)