SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    662, 666, 663, 348, 661, 350, 719, 313, 
    665, 659, 307, 398
  ) 
  AND cscart_product_descriptions.lang_code = 'fr'

Query time 0.00026

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_descriptions",
      "access_type": "range",
      "possible_keys": ["PRIMARY", "product_id"],
      "key": "product_id",
      "key_length": "9",
      "used_key_parts": ["product_id", "lang_code"],
      "rows": 12,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (662,666,663,348,661,350,719,313,665,659,307,398) and cscart_product_descriptions.lang_code = 'fr'"
    }
  }
}