SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20189, 20190, 20191, 20192, 20193, 20194, 
    20195, 20196, 20197, 20198, 20199, 
    20200, 20201, 20202, 20203, 20204
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00113

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 (20189,20190,20191,20192,20193,20194,20195,20196,20197,20198,20199,20200,20201,20202,20203,20204) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}