SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20125, 20126, 20127, 20128, 20129, 20130, 
    20131, 20132, 20133, 20134, 20135, 
    20136, 20137, 20138, 20139, 20140
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00092

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 (20125,20126,20127,20128,20129,20130,20131,20132,20133,20134,20135,20136,20137,20138,20139,20140) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}