SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21583, 21584, 21585, 21586, 21587, 21588, 
    21589, 21590, 21591, 21592, 21593, 
    21594, 21595, 21596, 21597, 21598
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00107

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 (21583,21584,21585,21586,21587,21588,21589,21590,21591,21592,21593,21594,21595,21596,21597,21598) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}