SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.meta_keywords, 
  cscart_product_descriptions.meta_description, 
  cscart_product_descriptions.search_words, 
  cscart_product_descriptions.promo_text, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    12520, 12497, 12528, 12512, 12511, 12527, 
    10580, 10579, 9931, 12521, 12504, 12502, 
    12505, 13099, 12514, 12513, 9759, 10496, 
    10560, 10561
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00055

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_descriptions",
      "access_type": "range",
      "possible_keys": ["PRIMARY", "product_id"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["product_id", "lang_code"],
      "rows": 20,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (12520,12497,12528,12512,12511,12527,10580,10579,9931,12521,12504,12502,12505,13099,12514,12513,9759,10496,10560,10561) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}