SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21567, 21568, 21569, 21570, 21571, 21572, 
    21573, 21574, 21575, 21576, 21577, 
    21578, 21579, 21580, 21581, 21582
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00067

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 (21567,21568,21569,21570,21571,21572,21573,21574,21575,21576,21577,21578,21579,21580,21581,21582) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}