SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20573, 20574, 20575, 20576, 20577, 20578, 
    20579, 20580, 20581, 20582, 20583, 
    20584, 20585, 20586, 20587, 20588
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00039

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 (20573,20574,20575,20576,20577,20578,20579,20580,20581,20582,20583,20584,20585,20586,20587,20588) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}