SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20733, 20734, 20735, 20736, 20737, 20738, 
    20739, 20740, 20741, 20742, 20743, 
    20744, 20745, 20746, 20747, 20748
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00103

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_descriptions",
      "access_type": "range",
      "possible_keys": ["PRIMARY", "product_id", "idx_product_lang"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["product_id", "lang_code"],
      "rows": 16,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (20733,20734,20735,20736,20737,20738,20739,20740,20741,20742,20743,20744,20745,20746,20747,20748) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}