At the moment, 6630 seems to be the only Nokia with "Feature Pack 2", that should grant Japanese text encoding, e-mail push and other features.
On Nokia Japan web site I have seen a Japanese 3G provider is actually selling this mobile.
Considering I would like to send and receive Japanese written e-mail, 6630 appears to be the only suitable device. I live in the EU, so I wonder if 6630 "EU version" allows to show up Japanese characters. Does it really make me able to manage Japanese text?
Or do I need to update software with a so-called Japanese version?
You need a version with a firmware release with Japanese language support. I doubt that any European Nokia resellers (retailers or operators) have such available.