How does the song go in your language? I only know it in English.

Happy birthday to you,
Happy birthday to you,
Happy birthday dear blankety blank,
Happy birthday to you!

Auf deutsch:

Häppi birsdäi tu ju
Häppi birsdäi tu ju
Häppi birsdäi, häppi birsdäi
Häppi birsdäi tu ju!

Italian:

Tanti auguri a te
Tanti auguri a te
Tanti auguri a [name]
Tanti auguri a te!

Portuguese (Portugal):

Parabéns a você
Nesta data querida
Muitas felizidades
Muitos anos de vida!
Hoje é dia de festa
Cantam as nossas almas
Para a/o menina/o [name]
Uma salva de palmas!

The common Brazilian version is quite different:
Parabéns pra você
Nesta data querida
Muitas felicidades
Muitos anos de vida!
Parabéns pra você
Nesta data querida
Muitas felicidades
Muitos anos de vida!
É pique! É pique! É pique, é pique, é pique!
É hora! É hora! É hora, é hora, é hora!
Rá-tim-bum!
(insert name)! (insert name)! (insert name)!
There are many regional variations, though, and even varaints used by certain devout Christians.

Haha, so it's longer than in most languages in Brazilian Portuguese too? In El Salvador's capital at least we usually sing an extended version that comes from a Mexican movie from the 50s (I think, I really don't know the origins). Also, in the extension the rhythm is the same.

Spanish:
Feliz cumpleaños a ti
Feliz cumpleaños a ti
Feliz cumpleaños (insert name)
Feliz cumpleaños a ti
[Extension]Ya queremos pastel
Ya queremos pastel
Aunque sea un pedacito
Pero queremos pastel

There's also another version, much less frequently sung:
Que los cumplas feliz
Que los cumplas feliz
Que los cumplas (insert name)
Que los cumplas feliz
http://www.youtube.com/watch?v=fWJRNJC0 ... re=related

Chinese:
祝你生日快樂/祝你生日快乐/zhù nǐ shēng rì kuài lè
祝你生日快樂/祝你生日快乐/zhù nǐ shēng rì kuài lè
祝你生日快樂/祝你生日快乐/zhù nǐ shēng rì kuài lè
祝你永遠快樂/祝你永远快乐/zhù nǐ yǒngyuǎn kuài lè
http://mandarin.about.com/library/audio ... inging.mp3

Yup. Some of the other versions are even longer, apparently.

Latin:

Felix natalis tibi.
Felix natalis tibi.
Felix natalis carus/cara [nomen].
Felix natalis tibi.

Danish:

I have no bloody idea. How on earth one would fit fødselsdag (birthday) into a song is beyond me.

In the alternative version of Spanish it doesn't mention the word "birthday". Actually, the word birthday means something similar to "year fulfilment" but with a less formal tone of course. So the song literally translates to something like "[I hope] that you fulfil them happy".

It is probably similar to the Norwegian one:

Gratulerer med dagen*
Gratulerer med dagen
Gratulerer, kjære ...
Gratulerer med dagen
*"Dagen" is pronounced "dag'n" /da:n/

See? The word "fødselsdag" doesn't appear at all.
The Danish also have their own birthday song with a different melody (so do we Norwegians), but I can't remember how it goes.

what about Zum Geburtstag viel Glück