SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21021, 21022, 21023, 21024, 21025, 21026, 
    21027, 21028, 21029, 21030, 21031, 
    21032, 21033, 21034, 21035, 21036
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00037

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 (21021,21022,21023,21024,21025,21026,21027,21028,21029,21030,21031,21032,21033,21034,21035,21036) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}