$query1 = (new \yii\db\Query())
->select("id, category_id AS type, name")
->from('post')
->limit(10);
$query2 = (new \yii\db\Query())
->select('id, type, name')
->from('user')
->limit(10);
$query1->union($query2);
$translation = Translation::find()->where(...)->one;
//если не нашли перевод, то по-умолчанию
if(!$translation) {
$translation = Translation::find()->where(...)->one
}