SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21503, 21504, 21505, 21506, 21507, 21508, 
    21509, 21510, 21511, 21512, 21513, 
    21514, 21515, 21516, 21517, 21518
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00039

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 (21503,21504,21505,21506,21507,21508,21509,21510,21511,21512,21513,21514,21515,21516,21517,21518) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}