SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21261, 21262, 21263, 21264, 21265, 21266, 
    21267, 21268, 21269, 21270, 21271, 
    21272, 21273, 21274, 21275, 21276
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00039

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 (21261,21262,21263,21264,21265,21266,21267,21268,21269,21270,21271,21272,21273,21274,21275,21276) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}