SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21519, 21520, 21521, 21522, 21523, 21524, 
    21525, 21526, 21527, 21528, 21529, 
    21530, 21531, 21532, 21533, 21534
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00036

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 (21519,21520,21521,21522,21523,21524,21525,21526,21527,21528,21529,21530,21531,21532,21533,21534) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}