javascript:void(location.href='http://translate.google.com/translate?u='+location.href)
Now when you're viewing a Japanese site, just click the toolbar bookmark and you'll see Google's translation of the page.
[robg adds: This should work in any browser, and it should also work for any language that Google can translate, not just Japanese. You can see what languages are possible on Google's Language Tools page. This is a more generic version of this older hint, which did the same thing but required a hard-coded URL for each language.]

