The legend of the croissant. The Turks were trying to invade Vienna and the Austro-Hugarian empire and they failed. The Turks had the crescent moon as their symbol of Islam, they failed and the Austrians to taunt them, and to commemorate this great victory made their bread in the shape of a croissant or a cross and eat it as if eating the Turks.
Marty believes that this legend that is used to explain the origins of an everyday item. He ran across it on one of his visits to Austria and was told this by a tour guide who had apparently been told about this legend as a child growing up in Austria.
One could assume that it is something like a legend of George Washington cutting down a cherry tree. The truth may not be there, but the legend is there to explain why something exists or why someone is the way they are.
This type of folklore seems to be more local and at the least very nationalistic. It does not seem likely that there are going to be a lot of people in Australia or New Zealand that are going to know about this legend, on the other hand it seems like people of Islamic faith, or of Austrian heritage are going to be more likely to know about this legend.
This legend also seems to be used to preserve the memory of a great moment in Austrian history. A moment where Austria was able to defend itself from invasion and maybe the reason why Marty was told this legend in Austria.