SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21293, 21294, 21295, 21296, 21297, 21298, 
    21299, 21300, 21301, 21302, 21303, 
    21304, 21305, 21306, 21307, 21308
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00057

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 (21293,21294,21295,21296,21297,21298,21299,21300,21301,21302,21303,21304,21305,21306,21307,21308) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}