'Buon cinquantesimo compleanno' or 'Auguri per il tuo cinquantesimo compleanno' would be two phrases you could use to wish someone a happy birthday in Italian.
The number fifty in Italian is cinquanta, and the ordinal number 'fiftieth' would be written as cinquantesimo. Ordinal numbers in Italian are marked with a small circle above the last digit like this: ° being equivalent to the English -th
So 50° would be the Italian numerical form of cinquantesimo, and might be found on an Italian fiftieth birthday cake.
To read more about wishing someone a 'happy birthday' in Italian, and to learn how to sing it, why not check out this blurtit question?