If you want to launch Billings (or any other app for that matter) in a different language than your default language, you can use a free contextual menu made by Arizone Software - makers of iLocalize. Useful for when you use your Mac in English and you want to launch an app in French for example.

Get the plugin here:

http://www.arizona-software.ch/ilocalize/launchapp-plugin.html

P.S. You can change your localization settings in System Preferences -> International.