SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21277, 21278, 21279, 21280, 21281, 21282, 
    21283, 21284, 21285, 21286, 21287, 
    21288, 21289, 21290, 21291, 21292
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00069

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 (21277,21278,21279,21280,21281,21282,21283,21284,21285,21286,21287,21288,21289,21290,21291,21292) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}