SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21791, 21792, 21793, 21794, 21795, 21796, 
    21797, 21798, 21799, 21800, 21801, 
    21802, 21803, 21804, 21805, 21806
  ) 
  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 (21791,21792,21793,21794,21795,21796,21797,21798,21799,21800,21801,21802,21803,21804,21805,21806) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}