To select the language to use while administering a blog, go to the admin page, and hit “Settings” – scroll down that page to the bottom, and you’ll see this language selector:

Languages available for the WordPress administration interface

For now, I’ve only added a handful of languages, but if I’ve missed a language that you want to use, please let me know and I’ll add it ASAP.

It’s not perfect, and not every bit of text is displayed in the selected language, but it is much better than a non-localizable admin interface. Here’s what the admin interface of my blog looks like with Chinese selected:

My blog's admin interface with Chinese selected as the active language.

Of course, the actual content has to be written in whatever language you need, and this won’t do any translation, but it should help by providing the administrative interface in various languages.