SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21775, 21776, 21777, 21778, 21779, 21780, 
    21781, 21782, 21783, 21784, 21785, 
    21786, 21787, 21788, 21789, 21790
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00036

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 (21775,21776,21777,21778,21779,21780,21781,21782,21783,21784,21785,21786,21787,21788,21789,21790) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}