SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    19901, 19902, 19903, 19904, 19905, 19906, 
    19907, 19908, 19909, 19910, 19911, 
    19912, 19913, 19914, 19915, 19916
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00033

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 (19901,19902,19903,19904,19905,19906,19907,19908,19909,19910,19911,19912,19913,19914,19915,19916) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}