To say happy belated birthday in the French language you can use the phrase "joyeux anniversaire tardif". It's a little formal though, so you may simply want to say "joyeux anniversaire un peu en retard" (Happy Birthday a little late).
The video below shows you how to pronounce happy birthday in French authentically.
If you do want to use the word "tardif", then this video shows you how to pronounce it properly:
Alternative ways to wish someone a happy birthday
Although the above way to say happy belated birthday in French is more universally used, there is also another accepted way of saying is which is: "Bon anniversaire tardif"
If you're speaking with native French speakers though, this phrase might mark you out as a foreigner, as it is hardly ever used!