SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    19581, 19582, 19583, 19584, 19585, 19586, 
    19587, 19588, 19589, 19590, 19591, 
    19592, 19593, 19594, 19595, 19596
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00137

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 (19581,19582,19583,19584,19585,19586,19587,19588,19589,19590,19591,19592,19593,19594,19595,19596) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}