SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20957, 20958, 20959, 20960, 20961, 20962, 
    20963, 20964, 20965, 20966, 20967, 
    20968, 20969, 20970, 20971, 20972
  ) 
  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 (20957,20958,20959,20960,20961,20962,20963,20964,20965,20966,20967,20968,20969,20970,20971,20972) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}