uilanguage, locale, none
When set to uilanguage, Setup will determine the default language to use by checking the user's "UI language" (by calling GetUserDefaultUILanguage(), or on Windows versions where that function is unsupported, by reading the registry). This is the method that Microsoft recommends. The "UI language" is the language used in Windows' own dialogs. Thus, on an English edition of Windows, English will be the default, while on a Dutch edition of Windows, Dutch will be the default. On the MUI edition of Windows, the default will be the currently selected UI language.
When set to locale, Setup will determine the default language to use by calling GetUserDefaultLangID(). This function returns the setting of "Your locale" in Control Panel's Regional Options. It should however be noted that the "Your locale" option is not intended to affect languages; it is only documented to affect "numbers, currencies, times, and dates".
When set to none, Setup will use the first language specified in the [Languages] section as the default language.