SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    21117, 21118, 21119, 21120, 21121, 21122, 
    21123, 21124, 21125, 21126, 21127, 
    21128, 21129, 21130, 21131, 21132
  ) 
  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 (21117,21118,21119,21120,21121,21122,21123,21124,21125,21126,21127,21128,21129,21130,21131,21132) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}