SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21759, 21760, 21761, 21762, 21763, 21764, 
    21765, 21766, 21767, 21768, 21769, 
    21770, 21771, 21772, 21773, 21774
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00065

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 (21759,21760,21761,21762,21763,21764,21765,21766,21767,21768,21769,21770,21771,21772,21773,21774) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}