不建议对入口文件index.php的修改，推荐的方法是在配置文件中（比如frontend/config/main.php）用behavior配置：

return [
    'id' => 'app-frontend',
    'basePath' => dirname(__DIR__),
    'bootstrap' => ['log'],
    'controllerNamespace' => 'frontend\controllers',
    // behavior
    'on beforeRequest' => function ($event) {
        $l_saved = null;
        if (true){
            # use cookie to store language
            $l_saved = Yii::$app->request->cookies->get('locale');
        }else{
            # use session to store language
            $l_saved = Yii::$app->session['locale'];
        }
        $l = ($l_saved)?$l_saved:'en-US';

        Yii::$app->sourceLanguage = 'en';
        Yii::$app->language = $l;
        return; 
    },    
    // 组件
    'components' => [
        'user' => [
    ......

处理多语言设置的action处

/**
* 设定语言： 1) 设置cookie，2) 跳转回原来的页面
* 访问网址 - http://.../site/language?locale=zh-CN
* @return [type] [description]
*/
public function actionLanguage()
{
	$locale = Yii::$app->request->get('locale');
	if ($locale){
		#use cookie to store language
		$l_cookie = new yii\web\Cookie(['name' => 'locale', 'value' => $locale, 'expire' => 3600*24*30,]);
		$l_cookie->expire = time() + 3600*24*30;
		Yii::$app->response->cookies->add($l_cookie);
	}
	$this->goBack(Yii::$app->request->headers['Referer']);
}

入口文件在高级版本是init之后产生的，默认不会被放到git库。