I have recently been exposed to this pokemon's official pronunciation and I refuse to accept it.

If the first part of Diancie's name should follow the pattern of 'diamond' (daJmənd), then it would surely be daJnsi. The random insertion of æ to make it daJænsi is stupid.

Since the above is all an ugly clusterfuck, I will continue to say diænsi just like I will continue to say Dialga like diælgə.