In Windows XP, Internet Explorer only provided partial tags for some languages. French (France) was "fr" and Japanese was "ja", yet English was "en-US".
In Windows Vista, IE7 is now using full locale names for the language, so HTTP Accept Language should now provide more complete information. (ie: ja-JP for Japanese, fr-FR for French (France)).
IE7 will also enumerate all of the system locales in the Language Preferences dialog, so custom locales / cultures such as haw-US, fj-FJ or even tlh-US would be listed for the user to select. Of course the user can still enter their own language tag if the desired locale isn't on the list.