SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21213, 21214, 21215, 21216, 21217, 21218, 
    21219, 21220, 21221, 21222, 21223, 
    21224, 21225, 21226, 21227, 21228
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00034

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 (21213,21214,21215,21216,21217,21218,21219,21220,21221,21222,21223,21224,21225,21226,21227,21228) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}