SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21455, 21456, 21457, 21458, 21459, 21460, 
    21461, 21462, 21463, 21464, 21465, 
    21466, 21467, 21468, 21469, 21470
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00052

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 (21455,21456,21457,21458,21459,21460,21461,21462,21463,21464,21465,21466,21467,21468,21469,21470) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}