SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20909, 20910, 20911, 20912, 20913, 20914, 
    20915, 20916, 20917, 20918, 20919, 
    20920, 20921, 20922, 20923, 20924
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00955

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 (20909,20910,20911,20912,20913,20914,20915,20916,20917,20918,20919,20920,20921,20922,20923,20924) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}