If you’re second guessing how Spanish speakers say “happy,” don’t bother Googling it.
The simple question itself, “how do you say happy cinco de mayo in spanish” yields a translator result that you could probably come up with yourself.
Okay, so you’re using part English, part Spanish – is that the problem?
Odds are, that’s probably not what you were looking for.
Saying “Happy Cinco de Mayo” will get your point across just fine, but just in case — “Feliz Cinco de Mayo.”