AdminCentral user interface for content authors is available in 16 languages which are shipped with the product: Chinese (Simplified), Chinese (Traditional), Czech, Dutch, English, French, German, Indonesian, Italian, Japanese, Norwegian, Portuguese, Portuguese (Brazil), Russian, Spanish and Swedish.
To set a user's language, go to Security and edit their profile. Select a language from the Language dropdown.
Internationalization effort translates the user interface to new languages. If you want Magnolia CMS in your own language, contribute to this effort.