SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21695, 21696, 21697, 21698, 21699, 21700, 
    21701, 21702, 21703, 21704, 21705, 
    21706, 21707, 21708, 21709, 21710
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00047

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 (21695,21696,21697,21698,21699,21700,21701,21702,21703,21704,21705,21706,21707,21708,21709,21710) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}