Not a myth–this is true! A quick check of the OED shows the origin of this English word is French for “Help me!” The Oxford English Dictionary says it is a phonetic reproduction of the French that has become an international signal of distress. The International Radio Telegraph Convention of 1927 lists 51 Rules, and this is among them. So the word “mayday” was officially born in 1927.
As one alert reader pointed out, the French is wrong. “Help me!” in French would be “Aidez-moi!” But then, Marconi was Italian. And if it wasn’t his fault, the International Radio Telegraph Convention was held in Washington, where French was not exactly a second language. So we’ll overlook the grammar.