SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21359, 21360, 21361, 21362, 21363, 21364, 
    21365, 21366, 21367, 21368, 21369, 
    21370, 21371, 21372, 21373, 21374
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00038

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 (21359,21360,21361,21362,21363,21364,21365,21366,21367,21368,21369,21370,21371,21372,21373,21374) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}