SELECT 
  product_id, 
  short_description, 
  full_description 
FROM 
  cscart_product_descriptions 
WHERE 
  product_id IN (13, 187, 16, 78, 520, 521) 
  AND lang_code = 'ru'

Query time 0.00068

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": 6,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (13,187,16,78,520,521) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}