SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    19789, 19790, 19791, 19792, 19793, 19794, 
    19795, 19796, 19797, 19798, 19799, 
    19800, 19801, 19802, 19803, 19804
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00040

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 (19789,19790,19791,19792,19793,19794,19795,19796,19797,19798,19799,19800,19801,19802,19803,19804) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}