Verified answer

as far as i know, you can't change the base language of the game, so if you have an english copy of diamond, the default names are all going to be in english.

You can give your pokemon's nicknames though and you can use the katakana equivalent of the english name, but then when it evolves it will keep that name that you nicknamed it.

the only possible way to alter the english game for japanese would be a hack or rom. so if you like the japanese names just give it to them as a nickname

Pokemon Diamond Edition

Go To Name Rater and change his name in english