SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20989, 20990, 20991, 20992, 20993, 20994, 
    20995, 20996, 20997, 20998, 20999, 
    21000, 21001, 21002, 21003, 21004
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00046

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 (20989,20990,20991,20992,20993,20994,20995,20996,20997,20998,20999,21000,21001,21002,21003,21004) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}