"Doğum günün kutlu olsun" is like "have a nice birthday". As you can see there is a verb(have) in the sentence.
"Doğum günün kutlu" is not a complete sentence for turkish. If you want to say something like "happy new years, happy birthday" you should say: "nice yıllara" or "mutlu yıllar". ("nice" means "lots" in here. so you can say that "nice yıllara" means "lots of years to live".) I hope this was not complicated and it is helpful for you :)