SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21101, 21102, 21103, 21104, 21105, 21106, 
    21107, 21108, 21109, 21110, 21111, 
    21112, 21113, 21114, 21115, 21116
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00029

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 (21101,21102,21103,21104,21105,21106,21107,21108,21109,21110,21111,21112,21113,21114,21115,21116) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}