In November 2006, my colleague Michael Schiltz and I have been working on Japanese and Dutch locales for the excellent Zotero Firefox extension. The latest Zotero release (= 1.0.0b3.r1) features our JA and NL translations of the Zotero UI (*), so go check it out.
For those new to Zotero, it's a "Next-Generation Research Tool" that "helps you collect, manage, and cite your research sources." If that sounds a bit opaque, have a look at the screencast demo, which does a great job explaining what Zotero is all about.
(*) I'd like to point out that our UI translation efforts so far were 1.0.0b2.r2 specific, so expect to see English strings for recently added functionality, such as the tagging interface in Zotero's left pane, the new report feature, etc. We'll do our best to have a complete translation ready for the next release.
Japanese and Dutch locale corrections, alternative translations and additions are absolutely welcome: feel free to leave a comment here or over at chosaq.