SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21197, 21198, 21199, 21200, 21201, 21202, 
    21203, 21204, 21205, 21206, 21207, 
    21208, 21209, 21210, 21211, 21212
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00928

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 (21197,21198,21199,21200,21201,21202,21203,21204,21205,21206,21207,21208,21209,21210,21211,21212) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}