SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    8987, 8990, 8992, 8993, 8994, 8996, 8997, 
    8999, 9003, 9004, 9005, 9006, 9009, 
    9011, 9020, 9021
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00031

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 (8987,8990,8992,8993,8994,8996,8997,8999,9003,9004,9005,9006,9009,9011,9020,9021) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}