At birthday parties in Romania the celebratory song of choice is Multi Ani Traiasca. The melody is identical to the German birthday song Hoch Soll Sie Leben and I suspect it came into use in Romania during the time of the Austro-Hungarian empire.
In the US the birthday song is Happy Birthday To You, a copyright-quagmire repurposing of the kindergarden song Good Morning To You.
Both birthday songs are 8 bars long and they stack on top of each other quite nicely when offset by one measure. So here's a short polyphonic arrangement of the two songs I wrote for a friend's birthday that will delight all your Romanian/German-American friends.