SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20669, 20670, 20671, 20672, 20673, 20674, 
    20675, 20676, 20677, 20678, 20679, 
    20680, 20681, 20682, 20683, 20684
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00041

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 (20669,20670,20671,20672,20673,20674,20675,20676,20677,20678,20679,20680,20681,20682,20683,20684) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}