SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21391, 21392, 21393, 21394, 21395, 21396, 
    21397, 21398, 21399, 21400, 21401, 
    21402, 21403, 21404, 21405, 21406
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00036

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 (21391,21392,21393,21394,21395,21396,21397,21398,21399,21400,21401,21402,21403,21404,21405,21406) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}