SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21005, 21006, 21007, 21008, 21009, 21010, 
    21011, 21012, 21013, 21014, 21015, 
    21016, 21017, 21018, 21019, 21020
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00067

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 (21005,21006,21007,21008,21009,21010,21011,21012,21013,21014,21015,21016,21017,21018,21019,21020) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}