SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21647, 21648, 21649, 21650, 21651, 21652, 
    21653, 21654, 21655, 21656, 21657, 
    21658, 21659, 21660, 21661, 21662
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

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 (21647,21648,21649,21650,21651,21652,21653,21654,21655,21656,21657,21658,21659,21660,21661,21662) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}