SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21165, 21166, 21167, 21168, 21169, 21170, 
    21171, 21172, 21173, 21174, 21175, 
    21176, 21177, 21178, 21179, 21180
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00084

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 (21165,21166,21167,21168,21169,21170,21171,21172,21173,21174,21175,21176,21177,21178,21179,21180) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}