SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20013, 20014, 20015, 20016, 20017, 20018, 
    20019, 20020, 20021, 20022, 20023, 
    20024, 20025, 20026, 20027, 20028
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00043

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 (20013,20014,20015,20016,20017,20018,20019,20020,20021,20022,20023,20024,20025,20026,20027,20028) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}