Proficiat!
You can also use these phrases, which convey different shades of meaning:
* Gefeliciteerd! (More formal than "Proficiat!")
* Van harte gefeliciteerd! (Means "Congratulations from the heart," expressing sincere joy)
* Proficiat met ... (Means "Congratulations on ...," followed by the specific achievement)
For example:
* Proficiat met je verjaardag! (Congratulations on your birthday!)
* Proficiat met je nieuwe baan! (Congratulations on your new job!)