The word "luau" itself is already Hawaiian and refers to the traditional Hawaiian feast.
To specify that it's a Hawaiian luau, you can use the phrase "ʻAhaʻaina Hawaiʻi" (pronounced ah-ha-eye-nah ha-why-ee). This translates to "Hawaiian feast" and more accurately reflects the traditional nature of the event.
You could also say "Lūʻau Hawaiʻi" (pronounced loo-ah-oo ha-why-ee), which directly combines the word "luau" with "Hawaiian."
Both phrases are grammatically correct and understandable in Hawaiian.