The patron interface for Ebook Central is available in eleven languages:
Czech - čeština
French - Français
Japanese - 日本語
Portuguese - Português
Chinese (simplified) - 简体中文
Chinese (traditional) - 繁體中文
English
German - Deutsch
Korean - 한국의
Spanish - Español
Turkish - Türkçe

To set your preferred language for the patron interface, select  User-added image from the top-right of the screen, then select "Language".  Click the radio button to select the desired language.  Then click "Save Changes"