I write it as: "Happy Birthday!"

In Slovene: Vse najboljše za tvoj rojstni dan. (we don't say happy birthday as in english; instead we say: All best for your birthday)

Lithuanian: Sveikinu su gimimo diena

In Italian "Buon Compleanno!" :)

In Slovak: "Pij viac!!" (Joking; that means "drink more"... the proper Slovak wish, as written on cards, is "Želám ti všetko najlepšie k narodeninám", "I wish you all the best for your birthday".) In Irish: "Breithlá Sona Duit!" ("Happy Birthday to you!")