SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21149, 21150, 21151, 21152, 21153, 21154, 
    21155, 21156, 21157, 21158, 21159, 
    21160, 21161, 21162, 21163, 21164
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00164

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 (21149,21150,21151,21152,21153,21154,21155,21156,21157,21158,21159,21160,21161,21162,21163,21164) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}