How do you say Sweet Sixteen in Japanese?

There isn't a direct translation for "Sweet Sixteen" in Japanese that captures the same cultural nuance. The closest you could get would be to translate it literally, which would sound unnatural:

* スイート シックスティーン (Suīto Shikkusutīn) This is a transliteration, simply using the English words in Japanese pronunciation. It would be understood, but sound foreign.

To convey the meaning of a 16th birthday celebration, you would be better off saying something like:

* 16歳の誕生日 (jūroku-sai no tanjōbi): 16th birthday

* 十六歳の誕生日パーティー (jūroku-sai no tanjōbi pātii): 16th birthday party

These options are much more natural and commonly used. The "sweet" aspect would need to be conveyed through the context of the celebration itself (e.g., describing it as a lovely or special party).