Under Administration, Locales And Themes, Default Locale is set to "en_US", or "fr". When a new user is created, and that user looks under "My Profile" it has a Default Locale of English (United Kingdom). Looking under user management doesn't show the field for defaultLocale until it's changed by the user under "My Profile". Until then, it uses English (United Kingdom) per "My Profile". At the bottom of all screens where you can choose the language it says it's using English (United States), which is correct. Under "My Profile" the Default Locale the implementation is using should be displayed properly even if the user hasn't manually selected a profile.
This happens in Version: 1.8.0 RC Build 19572.