SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20781, 20782, 20783, 20784, 20785, 20786, 
    20787, 20788, 20789, 20790, 20791, 
    20792, 20793, 20794, 20795, 20796
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00062

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 (20781,20782,20783,20784,20785,20786,20787,20788,20789,20790,20791,20792,20793,20794,20795,20796) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}