SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21743, 21744, 21745, 21746, 21747, 21748, 
    21749, 21750, 21751, 21752, 21753, 
    21754, 21755, 21756, 21757, 21758
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00116

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 (21743,21744,21745,21746,21747,21748,21749,21750,21751,21752,21753,21754,21755,21756,21757,21758) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}