SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20797, 20798, 20799, 20800, 20801, 20802, 
    20803, 20804, 20805, 20806, 20807, 
    20808, 20809, 20810, 20811, 20812
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00071

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 (20797,20798,20799,20800,20801,20802,20803,20804,20805,20806,20807,20808,20809,20810,20811,20812) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}