SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21727, 21728, 21729, 21730, 21731, 21732, 
    21733, 21734, 21735, 21736, 21737, 
    21738, 21739, 21740, 21741, 21742
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00031

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 (21727,21728,21729,21730,21731,21732,21733,21734,21735,21736,21737,21738,21739,21740,21741,21742) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}