Yesterday when I changed the site language to Japanese and browsed around, I noticed some menus and other navigational messages hadn't been translated. Today, somehow I already see some improvements. But things like "Customized Google", "Quick edit a Wiki page", "create/edit", "serach by name", "Maps feature is not correctly setup : Maps path is missing." are still in English. Although I don't think these would cause too much problems to Japanese editors (it's not like they are completely clueless as to reading English), it's not optimally welcoming, in my opinion.
To translate menus (just about anything other than main-body text) you need to use 'Edit languages'. Choose the language from the drop down list and click the edit button. You'll get a translation table. Many items (such as 'Customized Google') have not been translated. You can add new items and translate them yourself.