[JohnBraden]

You should bring a gift if they are inviting you to their home.
Alcohol should be okay-a bottle of whiskey perhaps.
I would certainly dress nicely, I don't know if a suit will be necessary.
Of course you should try your Japanese, provided you tell them it's not the best. They will understand and in turn tell you it's better than you think. (trying to be polite) or they will speak English with you. (If he's in the diplomatic corps, then at least he should be able to speak English)
Treat it as an honor to be invited and let them know that and you should be fine. Enjoy!