backend/views/bg/_form_language.php

<?php
    use common\models\BgLang;
    use common\modules\language\models\Language;
    use yii\web\View;
    use yii\widgets\ActiveForm;
    
    /**
     * @var BgLang     $model_lang
     * @var Language   $language
     * @var ActiveForm $form
     * @var View       $this
     */
?>
<?= $form->field($model_lang, '[' . $language->id . ']title')
         ->textInput([ 'maxlength' => true ]); ?>