The easy way to say happy birthday in Spanish is:
Feliz cumpleaños. — Happy birthday.
But, not so fast; what if it’s a “sweet sixteen” or, the very important coming-of-age party for a Latin American girl—her 15th birthday? Well, if it’s the latter, it’s called a quinceañera and it means fifteenth birthday celebration. If you want to wish her well, you would say:
Te deseo una quinceañera feliz. — Have a happy fifteenth birthday party (celebration).
What if you want to include their age in the greeting? For example, you want to say: “Happy 19th birthday.” Well, that’s a trick question and I’ll show you why.
If you enter “Happy nineteenth birthday” into one of those language translators, you’ll get back something like this: Feliz cumpleaños decimonoveno.
Here’s the problem with that: decimonoveno is the literal—ordinal—translation. In the real world, Spanish ordinal numbers (first, second, third…nineteenth, etc…) are rarely used after ten. In the case of the nineteenth birthday, you would instead say:
Feliz cumpleaños del número diecinueve. — Happy birthday [of the] number nineteen.
Feliz cumpleaños del diecinueve. — Happy birthday [of the] nineteen.
Now, if the celebrant will be (or became) ten or younger, here are the Spanish ordinal numbers you could use:
primero(a) — first
segundo(a) — second
tercero(a) — third
cuarto(a) — forth
quinto(a) — fifth
sexto(a) — sixth
séptimo(a) — seventh
octavo(a) — eighth
noveno(a) — ninth
décimo(a) — tenth
¿Fue el quinto cumpleaños de María? — Was it Maria's fifth birthday?
Two more rules for Spanish ordinals:
1. Ordinals must agree in number and gender of the nouns they modify. For example:
Es la quinta vez que te llamo hoy. — This is the fifteenth time I'm calling you today.
2. The “o” is dropped from primero and tercero before masculine singular nouns, for example:
el primer día del mes — the first day of the month
Back to birthdays:
Hoy celebramos el primer cumpleaños del bebé. — Today we celebrate the baby's first birthday.
¡Cuando te toca el tuyo, feliz cumpleaños!