SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20509, 20510, 20511, 20512, 20513, 20514, 
    20515, 20516, 20517, 20518, 20519, 
    20520, 20521, 20522, 20523, 20524
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00037

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 (20509,20510,20511,20512,20513,20514,20515,20516,20517,20518,20519,20520,20521,20522,20523,20524) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}