From this site

Traditional Spanish Happy Birthday Lyrics

Although the above song is gradually becoming the more accepted song for singing at birthdays in Spain, the Spanish do have a song of their own which has traditionally been sung at birthdays and is still very much in use to-day.

Here it is, followed by an English translation.

Feliz, feliz en tu día,
Amiguito(a) qué Dios te bendiga,
Y qué reine la paz en tu día,
Y qué cumplas muchos más.

Happy, happy on your day,
Friend, may God bless you,
May peace reign on your day,
And may you have/complete many more (birthdays, years).