After a lot of testing and following the advices from the other answers, I've finally fixed the issue.
Issues with WebView:
- WebViewhave to be loaded dynamically in the activity. Loading from XML file via- Activity#setContentView()won't work for the first time and localisation of the entire activity will break. However, subsequent launches or- Activity#recreate()will work as expected.
- Even if WebViewis loaded dynamically, it won't work for the first time. Again, subsequent launches orActivity#recreate()will work properly.
Considering the issues above, the solution involves loading WebView dynamically and then, the immediate recreation of the activity.
ActivityWithWebView.java
public class ActivityWithWebView extends AppCompatActivity {
    private WebView webView;
    private static boolean firstTime = true;
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        setContentView(R.layout.activity_with_web_view);
        setSupportActionBar(findViewById(R.id.toolbar));
        ActionBar actionBar = getSupportActionBar();
        if (actionBar != null) actionBar.setTitle(R.string.instructions);
        // WebView has to be loaded dynamically to prevent in-app localisation issue.
        webView = new WebView(this);
        if (firstTime) {
            // Recreate if loaded for the first time to prevent localisation issue.
            recreate();
            firstTime = false;
            return;
        }
        LinearLayoutCompat webviewWrapper = findViewById(R.id.webview_wrapper);
        webView.setLayoutParams(new ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,
                ViewGroup.LayoutParams.WRAP_CONTENT));
        webviewWrapper.addView(webView);
        // Do other works.
    }
}
activity_with_web_view.xml
<?xml version="1.0" encoding="utf-8"?>
<androidx.coordinatorlayout.widget.CoordinatorLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <com.google.android.material.appbar.AppBarLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:id="@+id/appbar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">
        <androidx.appcompat.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?android:colorBackground" />
    </com.google.android.material.appbar.AppBarLayout>
    <androidx.appcompat.widget.LinearLayoutCompat
        android:id="@+id/webview_wrapper"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_behavior="com.google.android.material.appbar.AppBarLayout$ScrollingViewBehavior" />
</androidx.coordinatorlayout.widget.CoordinatorLayout>