If one is writing for the English speaking part of the world it should be Standart. [English is certainly the international language today.]
The name is derived from the flag. English speaking people generally shy away from words that sound to Germanic. Therefore, the yacht has come to be known as Standart, which sounds more acceptable to most English speaking people. The IF usually spoke English among themselves. The IF usually stayed away from words that sounded too German for obvious reason. Shtandart is too German sounding.
When in doubt, the International Lloyds Registration designation is the correct name to be followed, which lists the yacht as the Standart.
Main Entry: Standard
Function: noun
Date: 12th century
1 : a conspicuous object (as a banner) formerly carried at the top of a pole and used to mark a rallying point especially in battle or to serve as an emblem