If you are supporting a Japanese "community" style site,
you don't want to let them submit japanese characters in
the email field. As Todd mentioned, the mail RFC says
double byte usernames before the "@" are theoretically
possible, but I've never actually seen this in practice...
everyone just uses ascii.
As Henry said, you will want to add some additional fields
for the katakana spellings (furigana) of people's names. I've found
that you will end up adding the following to the users table:
family_furigana_name
given_furigana_name
Then stick the kanji names in first_names and last_name.
You probably want to sort on family_furigana_name.
Also, keep in mind that last name comes first,
so you will want to adjust this in the UI.
One other thing, for double byte varchar and char fields,
you will want to triple the size of the field. This isn't as
much of an issue for the name fields, as names are only
1-3 kanji.