Have you looked at the language files for SMF (or phpBB ? )
http://download.simplemachines.org/?languages
http://www.phpbb.com/languages/
With SMF you can compare the string codes in the English and Japanese files to get interface translations for most things you might want to say.
In the phpBB files the English seems to be in the Japanese file itself, making it a bit easier to find stuff perhaps.
login forever is 期間限定無し according to SMF
or you might find that someone has already done a translation for whatever software you're using, or you might consider using one that is already translated and save yourself a lot of work.
(I chose SMF when I set up a BBS for a Japanese teacher, mainly because it had a Japanese translation so she could administer it without dealing with the complexities of computer English jargon. Maybe phpBB didn't have a Japanese translation at that time, I don't exactly remember.)