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I did this the hardway, but it does work perfectly. In my case, I did a standard install. The I downloaded the Japanese version and expanded onto disk, Then I opened the language files for every module,
e.g. \html\modules\system\language\english\modinfo.php.
I then added the second langauge translation immediately after the English in the constant definition. So it looks like this
define("_MI_SYSTEM_BNAME2","User Menu :ウサ メニウ");
Did that for almost everthing. It took a while but works perfectly. You have to make sure your theme has enough width to handle this and there are some other minor issues of look and feel of the site, but it does work.
Have a quick look
here.
This way users don't have to choose one or the other. Both are there. Just takes a little design planning. I'm still working on this so some of it is still in English only and some is both.
Hint: If you want to admin the site in English, then don't change the admin language files. Or just over right them with the other language files.
Upgrades are going to be a nightmare if they add next entries, but hey, I knew that when I started.