Language meta information should reflect actual application language (instead of hardcoded EN)

Currently, the layout of the basic and advanced app templates has English hardcoded as language:
```php
<html lang="EN">
```

Since language is a dynamic attribute in the application config. Based on Paul's suggestion, I propose to use this:
```php
<html lang="<?= str_replace('_', '-', Yii::$app->language) ?>">
```