Creating a drop-down list input in Yii2 framework is simple like the following example
<?= $form->field($model, 'language_id')->dropDownList([1 => 'item 1', 2 => 'item 2']) ?>
<?= $form->field($model, 'language_id')->dropDownList([1 => 'item 1', 2 => 'item 2'], ['prompt' => 'Choose your option']) ?>
If your drop-down has many options and you want to enable the search in the list. You can use the great plugin of JQuery Select2 through the extension yii2-widget-select2