Admin Profile Locale Selector by Innobyte. By installing "Profile Locale Selector", your administrators can control the default language of your website's back-end. Very easy to install, this extension is developed and tested by certified Magento developers. "Profile Locale Selector" solves one of the most recurrent Magento issues: individually setting the default language to English (or any other preferred language).
Key features: Adds a new column in the admin_user table to store the locale for each user, defaults to en_US; Administrators can choose directly what language they want to use for back-end. Other extension features: Easy and friendly administration; Meets Magento programming practices.