>>38329638Considering GF chooses the names of box legendaries so that they don't change across different languages, this is a very strong evidence.
For those citing "diamond" as proof, Pokemon is NOT made with only americans in mind. The english version of this word comes from the old french "diamant", which is pronounced as "dee-ah". It's also like that in spanish and german.
It's the english pronunciation that is the odd one, they chose not to go with it.