SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21069, 21070, 21071, 21072, 21073, 21074, 
    21075, 21076, 21077, 21078, 21079, 
    21080, 21081, 21082, 21083, 21084
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00034

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 (21069,21070,21071,21072,21073,21074,21075,21076,21077,21078,21079,21080,21081,21082,21083,21084) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}